Source : Webster's Revised Unabridged Dictionary (1913)

Reentrant \Re*["e]n"trant\ (-trant), a.
   Re["e]ntering; pointing or directed inwardds; as, a re?ntrant
   angle.

Source : WordNet®

reentrant
     adj : (of angles) pointing inward; "a polygon with re-entrant
           angles" [syn: {re-entrant}] [ant: {salient}]

Source : Free On-Line Dictionary of Computing

re-entrant
     
         Used to describe code which can have multiple
        simultaneous, interleaved, or nested invocations which will
        not interfere with each other.  This is important for
        {parallel processing}, {recursive} functions or subroutines,
        and {interrupt handling}.
     
        It is usually easy to arrange for multiple invocations
        (e.g. calls to a subroutine) to share one copy of the code and
        any read-only data but, for the code to be re-entrant, each
        invocation must use its own copy of any modifiable data (or
        synchronised access to shared data).  This is most often
        achieved using a {stack} and allocating local variables in a
        new {stack frame} for each invocation.  Alternatively, the
        caller may pass in a pointer to a block of memory which that
        invocation can use (usually for outputting the result) or the
        code may allocate some memory on a {heap}, especially if the
        data must survive after the routine returns.
     
        Re-entrant code is often found in system software, such as
        {operating systems} and {teleprocessing monitors}.  It is also
        a crucial component of {multithreaded} programs where the term
        "thread-safe" is often used instead of "re-entrant".
     
        (1996-12-21)