The Collaborative International Dictionary of English v.0.48:

Reentrant \Re*en"trant\ (-trant), a.
   Reentering; pointing or directed inwards; as, a reentrant
   angle.
   [1913 Webster]

WordNet (r) 3.0 (2006):

re-entrant
    adj 1: (of angles) pointing inward; "a polygon with re-entrant
           angles" [syn: re-entrant, reentrant] [ant: salient]

The Free On-line Dictionary of Computing (30 December 2018):

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)