Atom
Nuclear Physics
A particle of matter indivisible by chemical means. It is the fundamental building block of elements.
- — Glossary of Nuclear Science Terms retrieved 2014-05-20
Perl
A "regular expression" component potentially matching a "substring" containing one or more characters and treated as an indivisible syntactic unit by any following "quantifier". (Contrast with an "assertion" that matches something of "zero width" and may not be quantified.)