leaf primordiumA lateral outgrowth from the apical meristem, which will become a leaf when fully developed and expanded.
leaf rollA symptom of some virus diseases, characterized by curling of the leaves. Can also occur as a response to water stress.
leaf scarMark left on a stem after leaf abscission.
leafletExpanded leaf-like part of a compound leaf.
LeakUncontrolled loss of water by flow through a hole or crack.
LeakageFree flow loss of water through a hole or crack.
Leaking ElectricityRelated to stand-by power, leaking electricity is the power needed for electrical equipment to remain ready for use while in a dormant mode or operation.
leaky mutantA mutant in which the gene product still retains some biological activity.
Leapfrog DevelopmentDevelopment that occurs beyond the limits of existing development and creates areas of vacant land between areas of developed land.
Leasable mineralsFederal minerals such as coal, oil shale, oil, gas, phosphate, potash, sodium, tar sands, geothermal resources, potassium, asphaltic materials, and all…
LeaseA contract transferring the use of property or occupancy of land, space, structures, or equipment in consideration of a payment (e.g., rent).
Lease of power privilegeContractual right given to a non-Federal entity to utilize, consistent with project purposes, water power head and storage from Reclamation projects for…
Least cost planning(integrated resource planning) A method of power planning that recognizes load uncertainty, embodies an emphasis on risk management, and reviews all…
lectinA complex molecule that has both protein and sugars. Lectins are able to bind to the outside of a cell and cause biochemical changes in it.
LeeThe side or part that is sheltered or turned away from the wind, such as with a mountain.
Lee WaveThe wavelike effect, characterized by severe updrafts and downdrafts, that occurs in the lee of a mountain range when rapidly flowing air is lifted up the…
LEED "Plus"Created by the Massachusetts Sustainable Design Roundtable to address shortcomings with standard LEED certification these standards apply to construction…
Leeside LowExtratropical cyclones that form on the downwind (lee) side of a mountain chain.
LeewardThe side away from the wind. Compare windward.
Left Exit RegionUsed interchangably with Left Front Quadrant; the area downstream from and to the left of an upper-level jet max (as would be viewed looking along the…
Left Front QuadrantUsed interchangably with Left Exit Region; the area downstream from and to the left of an upper-level jet max (as would be viewed looking along the…
Left MoverA thunderstorm which moves to the left relative to the steering winds, and to other nearby thunderstorms; often the northern part of a splitting storm.
Left or right designationIn the context of dams and river channels, the designation of left or right is made with the observer looking in the downstream direction.
LenticStill water systems such as lakes and reservoirs.
Lentic environmentAn aquatic ecosystem in which the water is still and not rapidly moving, such as is found in ponds and swamps.
Lentic SystemA nonflowing or standing body of fresh water, such as a lake or pond. Compare lotic system.
Lenticular CloudA very smooth, round or oval, lens-shaped cloud that is often seen, singly or stacked in groups, near or in the lee of a mountain ridge.
LeptinA hormone produced by the placenta and fetal tissues that acts as a growth factor and modulator of metabolic and immune functions.
leptonemaStage in meiosis immediately preceding synapsis and post DNA replication, in which the chromosomes appear as single, fine, threadlike structures.
leukemia(loo-KEE-mee-uh) Cancer that starts in blood-forming tissue such as the bone marrow and causes large numbers of blood cells to be produced and enter the…
Leukemia inhibitory factorA cell messenger protein originally noted for inhibition of mouse M1 myeloid leukemia cells that also has effects including inhibiting differentiation to…
Leukemia inhibitory factor (LIF)A growth factor necessary for maintaining mouse embryonic stem cells in a proliferative, undifferentiated state.
Leukocyte(LOO-koh-site) A white blood cell. Refers to a blood cell that does not contain hemoglobin.
Leukocyte (White Blood Cell)The general name for a white blood cell. Leukocytes are responsible for maintaining the immune system's response to foreign substances and infection.
Leukocyte common antigenCell-surface molecule found on white blood cells and white blood cell progenitors. Also referred to as CD45.
LeveeA natural or man-made barrier that helps keep rivers from overflowing their banks. See dike.
Levee (Dike)A long, low embankment usually built along a watercourse to protect land from flooding.
LevelThe number of values used in a card counting system. A level one counting system only uses +1 and -1, but a level two system would use +1, +2, -1 and -2…
Level of Free Convection(LFC) - The level at which a parcel of saturated air becomes warmer than the surrounding air and begins to rise freely.
Level of protection (LOP)As used in the evaluation process, it is the level of loading selected to which corrective actions will be designed to prevent dam failure.
Level of Service (LOS)(LOS)A qualitative measure describing operational conditions within a traffic stream in terms of speed and travel time, freedom to maneuver, traffic…
LEWPLine Echo Wave Pattern. A bulge in a thunderstorm line producing a wave-shaped "kink" in the line.
LFCAn acronym for Level of Free Convection- the level at which a parcel of saturated air becomes warmer than the surrounding air and begins to rise freely.
LH2OThe heat (Btu) needed to vaporize and superheat one pound of water.
LILifted Index. A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the ground would have if it…
libraryA collection of cells, usually bacteria or yeast, that have been transformed with recombinant vectors carrying DNA inserts from a single species.
Licensea contract in which the owner of the patent grants another person or company permission to make, use, or sell the patented invention.
Lid(Also called cap.) A region of negative buoyancy below an existing level of free convection (LFC) where energy must be supplied to the parcel to maintain…
LIDARAn instrument similar to a radar but uses a laser to generate light pulses rather than radio waves, generally used to measure aerosol content and particle…
life cycleThe sequence of events from a given developmental stage in one generation to the same stage in the following generation.
Life Cycle CostThe sum of all the costs both recurring and nonrecurring, related to a product, structure, system, or service during its life span or specified time…
Life-cycle costingA method of comparing costs of equipment or buildings based on original costs plus all operating and maintenance costs over the useful life of the…
Lifetime (Atmospheric)The lifetime of a greenhouse gas refers to the approximate amount of time it would take for the anthropogenic increment to an atmospheric pollutant…
LiftThe force that pulls a wind turbine blade, as opposed to drag.
Lift lineHorizontal construction joint created when new concrete is placed on previously placed concrete.
Lifted Index(abbrev. LI)- A common measure of atmospheric instability. Its value is obtained by computing the temperature that air near the ground would have if it…
Lifting Condensation Level(LCL) - The level at which a parcel of moist air becomes saturated when it is lifted dry adiabatically.
ligandA small molecule (e.g. activators, substrates and inhibitors of enzyme activity) bound to a protein by non-covalent forces; an ion or a molecule that…
Ligase (DNA ligase)An enzyme that catalyzes a condensation reaction that links two DNA molecules via the formation of a phosphodiester bond between the 3' hydroxyl and 5'…
ligase chain reaction(Abbreviation: LCR). A technique for the detection and amplification of target DNA sequences.
LigateThe process of joining two or more DNA fragments.
ligate, ligationThe joining of two linear double-stranded DNA fragments by the formation of phosphodiester bonds.
LightA pane of glass; a window, or a compartment of a window.
Light BridgeIn solar-terrestrial terms, it is observed in white light, a bright tongue or streaks penetra- ting or crossing sunspot umbrae.
Light fixtureAn electrical device used to create artificial light and/or illumination.
Light pollutionAny adverse effect of human-made lighting, such as excessive illumination of night-skies by artificial light.
Light QualityA description of how well people in a lighted space can see to do visual tasks and how visually comfortable they feel in that space.
Light spillageAn undesirable condition in which light is cast where it is not wanted. (Also referred to as light trespass.)
Light TrappingThe trapping of light inside a semiconductor material by refracting and reflecting the light at critical angles; trapped light will travel further in the…
Light YearThe distance light travels in a year: {Distance equals rate times time: 300,000,000 meters per second, times roughly 30 million seconds equals roughly} 9.
Light-emitting diode (LED)(LED)This very energy efficient lighting technology uses 80 to 90% less energy than conventional lights.
Light-Induced DefectsDefects, such as dangling bonds, induced in an amorphous silicon semiconductor upon initial exposure to light.
Lightning(abbrev. LTNG) A visible electrical discharge produced by a thunderstorm. The discharge may occur within or between clouds, between the cloud and air…
Lightning ChannelThe irregular path through the air along which a lightning discharge occurs. A typical discharge of flash between the ground and the cloud is actually a…
Lightning DischargeThe series of electrical processes by which charge is transferred along a channel of high ion density between electrical charge centers of opposite sign.