TMPDIR is the canonical Unix environment variable that should be used to specify a temporary directory for scratch space. Most Unix programs will honor this setting and use its value to denote the scratch area for temporary files instead of the common default of /tmp or /var/tmp.
Other forms sometimes accepted are TEMP, TEMPDIR and TMP, but these alternatives are used more commonly by non-POSIX operating systems or non-conformant programs.
TMPDIR is specified in various Unix and similar standards, e.g. per the Single UNIX Specification.