Common term for what is officially known as `
REVERSE SOLIDUS'. In
ASCII,
ISO 8859-N, and
Unicode. In Unicode, U+005C (92. decimal). Backslash is not necessarily in
ISO 646---in many
Scandinavian countries, the characters [\]{|} were
coöpted for use as letters (which, incidentally, is why
IRC considers them to be letters).
Backslash is used in many environments (C, Common Lisp, Perl, and nearly every Unix shell, to name a few) to quote special characters, or to introduce control characters. A few systems (most notably DOS, NT, and their descendents) use backslash as the path separator---note however, that they generally allow forward slash to be used for that purpose at the syscall level; it is simply a matter of getting the programs to not treat forward slash specially.