A unix command for searching files for lines matching a given regular expression (RE). Named after the qed/ed editor subcommand "g/re/p", where re stands for a regular expression, to Globally search for the Regular Expression and Print the lines containing matches to it. There are two other variants, fgrep which searches only for fixed strings and egrep which accepts extended REs but is usually the fastest of the three.
Used by extension to mean "to look for something by pattern". When browsing through a large set of files, one may speak of "grepping around". "Grep the bulletin board for the system backup schedule, would you?" See also vgrep.