Science terms starting with P

1,062 terms

pedicel Stalk or stem of the individual flowers of an inflorescence.
pedigree A record of one's ancestors, offspring, siblings, and their offspring that may be used to determine the pattern of certain genes or disease inheritance…
Pediment 1. The triangular space forming the gable end of a roof above the horizontal cornice. 2. An ornamental gable, usually triangular, above a door or window.
peduncle Stalk or stem of a flower that is born singly; the main stem of an inflorescence.
PEG Abbreviation for polyethylene glycol.
Pegmatite Very coarse-grained intrusive igneous rock of granitic composition that typically fills fractures to form veins.
Pelagic Pertaining to the open ocean and organisms living within it, including highly migratory fishes such as swordfish, tuna, and many species of shark.
Pellet Stove A space heating device that burns pellets; are more efficient, clean burning, and easier to operate relative to conventional cord wood burning appliances.
Pellets Solid fuels made from primarily wood sawdust that is compacted under high pressure to form small (about the size of rabbit feed) pellets for use in a…
Pelton Turbine A type of impulse hydropower turbine where water passes through nozzles and strikes cups arranged on the periphery of a runner, or wheel, which causes the…
Pendant Echo Radar signature generally similar to a hook echo, except that the hook shape is not as well defined.
Penetrance The proportion of individuals in a population that express the phenotype expected from their genotype with respect to a specific gene.
Penetrating Top Same as Overshooting Top; a dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and hence a higher potential for severe…
Penstock A component of a hydropower plant; a pipe that delivers water to the turbine.
Pentanes plus A mixture of hydrocarbons, mostly pentanes and heavier fractions, extracted from natural gas. See hydrocarbons.
Penumbra In solar-terrestrial terms, the sunspot area that may surround the darker umbra or umbrae.
peptidase An enzyme that catalyzes the hydrolysis of a peptide bond.
peptide Any compound consisting of two or more amino acids, the building blocks of proteins.
peptide bond The chemical bond holding amino acid residues together in peptides and proteins.
peptide expression library A collection of peptide molecules, produced by recombinant cells, in which the amino acid sequences are varied.
peptide nucleic acid (Abbreviation: PNA). A synthetic oligonucleotide analogue, in which the sugar backbone is replaced by a peptide chain, upon which the nucleoside residues…
peptide vaccine A short chain of amino acids that can induce antibodies against a specific infectious agent.
peptidyl transferase An enzyme bound tightly to the large sub-unit of the ribosome, that catalyses the formation of peptide bonds between amino acids during translation.
peptidyl-tRNA binding site (Abbreviation: P-site). The site on a ribosome that hosts the tRNA to which the next amino acid for the growing polypeptide chain is attached.
Per capita income The average income per person in a given group.
Per capita use The average amount of water used per person during a standard time period, generally per day.
Perched Groundwater Local saturated zones above the water table which exist above an impervious layer of limited extent.
Perched Water Table Underground water lying over dry soil, and sealed from it by an impervious layer.
Percolation The filtering of a liquid passed through a medium with many fine spaces.
Percolation Deep In irrigation or farming practice, the amount of water that passes below the root zone of the crop or vegetation.
Percolation Path The course followed by water moving or percolating through any other permeable material, or under a dam which rests upon a permeable foundation.
Percolation Rate The rate at which water moves through porous media, such as soil. The intake rate used for designing wastewater absorption systems.
perennial A plant that flowers continuously for several years.
Perennial allotment A BLM grazing allotment that consistently produces enough perennial forage to support a year round livestock operation.
Perennial Stream A stream that flows continually throughout the year.
Perennial streams Streams that flow continuously because they lie at or below the groundwater table, which constantly replenishes them.
Perennial surface water features Surface water features that contain water at all times throughout the year.
Perennial yield Maximum quantity of water that can be annually withdrawn from a ground water basin over a long period of time (during which water supply condditions…
Perennial yield (groundwater) The amount of groundwater that can be withdrawn from a groundwater basin over a period of time without exceeding the long-term recharge of the basin or…
Perennial/safe/sustainable yield A specified rate of groundwater pumping that can be sustained for an indefinite period of time without impairing hydrogeologic and ecologic processes…
Perfluorocarbon (PFC) Compounds consisting of carbon and fluorine. They do not deplete the stratospheric ozone but are very strong greenhouse gases with long lifetimes in the…
Perfluorocarbon Tracer Gas Technique (PFT) An air infiltration measurement technique developed by the Brookhaven National Laboratory to measure changes over time (one week to five months) when…
Perfluorocarbons (PFCs) A group of human-made chemicals composed of carbon and fluorine only. These chemicals (predominantly CF4 and C2F6) were introduced as alternatives, along…
Perforated pipe Pipe designed to discharge water through small, multiple, closely spaced orifices or nozzles, placed in a segment of its circumference for irrigation…
Performance Ratings Solar collector thermal performance ratings based on collector efficiencies, usually expressed in Btu per hour for solar collectors under standard test or…
Performance Zoning Establishes minimum criteria to be used when assessing whether a particular project is appropriate for a certain area; ensures that the end result adheres…
pericentric inversion A chromosomal rearrangement in which a segment of chromosome, including the centromere, is rotated.
periclinal The orientation of cell wall or plane of cell division parallel to the reference surface.
periclinal chimera 1. Genotypically or cytoplasmically different tissues arranged in concentric layers. 2.
pericycle Region of the plant bounded externally by the endodermis and internally by the phloem. Most roots originate from the pericycle.
Perigee The closest distance between moon and earth or the earth and sun.
Perihelion The point on the annual orbit of a body (about the sun) that is closest to the sun; at present, the earth reaches this point on about 5 January.
Perimeter Heating A term applied to warm-air heating systems that deliver heated air to rooms by means of registers or baseboards located along exterior walls.
Period Usually designated by T. This is the time interval of one complete oscillation of any wave. See Wavelength.
Periodic Facility Review (PFR) A field review performed on a high- or significant-hazard dam every 6 years that entails a thorough examination from both operation and maintenance and…
Peripheral Blood Stem Cell Transplant (PBSCT) A method of separating and collecting stem cells from the circulating blood stream using a process called pheresis.
Periphyton Microscopic plants and animals that are firmly attached to solid surfaces under water such as rocks, logs, pilings and other structures.
periplasm The space between the cell (cytoplasmic) membrane of a bacterium or fungus and the outer membrane or cell wall. Synonym: periplasmic space.
Permafrost Perennially frozen soil.
Permanence of Protection For the Marine Protected Areas Inventory, in order for sites to be considered for inclusion in the database, they must provide year round (12-month)…
Permanent Control A stream gaging control which is substantially unchanging and is not appreciably affected by scour, fill, or backwater.
Permanent Monument Fixed monuments placed away from the dam which allow movements in horizontal and vertical control points on the dam to be monitored by using accurate…
permanent wilting point (Abbreviation: PWP). The moisture content of a soil below which plants wilt to such an extent that they fail to recover even when fully watered.
Permanent wilting point (permanent wilting percentage) Soil water content below which plants cannot readily obtain water and permanently wilt. See wilting point.
Permeability The rate at which liquids pass through soil or other materials in a specified direction.
Permeability Coefficient The rate of flow of a fluid through a cross section of a porous mass under a unit hydraulic gradient, at a temperature of 60 degrees Fahrenheit.
Permeable Referring to a membrane, cell or cell system through which small molecules can diffuse.
Permeameter Device for containing the soil sample and subjecting it to fluid flow in order to measure permeability or hydraulic conductivity.
Permeance A unit of measurement for the ability of a material to retard the diffusion of water vapor at 73.4 F (23 C).
Permeate To penetrate and pass through, as water penetrates and passes through soil and other porous materials.
Permissible Exposure Limit (PPL) The maximum amount or concentration of a chemical that a worker may be exposed to under OSHA regulations.
Permit Any permit other than a Notice to Proceed issued by the Landmarks Preservation Commission in accordance with the Landmarks Law: (a) "PMW shall mean Permit…
Permitted use The forage allocated by, or under the guidance of, an applicable land use plan for livestock grazing in an allotment under a permit or lease; expressed in…
Permittee An individual who holds either a BLM grazing permit or grazing lease that authorizes grazing use of the public lands issued under authority of Section 3…
persistence Ability of an organism to remain in a particular setting for a period of time after it is introduced.
Persistence Forecast A forecast that the current weather condition will persist and that future weather will be the same as the present (e.g.
Persistent Chemicals with a long inactivation or degradation time, such as some pesticides.
Personal Protective Equipment (PPE) Clothing or devices worn to help isolate or shield a person from direct exposure to a hazardous material or situation.
Perturbation Model A computer model used to calculate air pollution concentrations. A perturbation model produces a wind field from solutions to a simplified set of…
PERV Abbreviation for porcine endogenous retrovirus.
Pervious Permeable, having openings that allow water to pass through.
Pervious Zone A part of the cross section of an embankment dam comprising material of high permeability.
Pesticide (PES-tih-side) Any substance that is used to kill insects and other pests.
Pesticide resistance A genetic change in response to selection by a pesticide, resulting in the development of strains capable of surviving a dose lethal to most individuals…
Pestle An elongated, often cylindrical stone used to pulverize food products and other cultural products in a mortar. See also Mortar
petal One of the parts of the flower that make up the corolla.
Petcock A small drain valve. A small ground key type valve used with soft copper tubing.
petiole Stalk of leaf. See: pedicel; peduncle.
Petri dish Flat round glass or plastic dish with a matching lid, used for small-scale culturing of organisms, germinating seeds etc.
Petrocalcic Soil horizon formed when secondary calcium carbonate accumulates in the subsoil and hardens into a hardpan.
Petrochemical feedstock Feedstock derived from petroleum, used principally for the manufacture of chemicals, synthetic rubber, and a variety of plastics.
Petrochemicals Chemicals obtained by refining (i.e., distilling) crude oil. They are used as raw materials in the manufacture of most industrial chemicals, fertilizers…
Petroglyph A figure or design carved, abraded, or pecked on rock.
Petroleum Any petroleum-based substance composed of a complex blend of hydrocarbons derived from crude oil, including motor fuel, jet oil, lubricants, petroleum…
Petroleum coke A residue that is the final product of the condensation process in cracking.
PETS Proposed, endangered, threatened, or sensitive species.
PFD A personal flotation device or life jacket.
PFGE Abbreviation for pulsed-field gel electrophoresis.
PFYC See Potential Fossil Yield Classification
PG One of several hormone-like substances made by the body. Different PGs control blood pressure, contraction of smooth muscles, and other processes within…