Definition from Wiktionary, the free dictionary
reentrant (not comparable)
- Reentering; pointing inward.
- (programming, of a mutual exclusion mechanism) Such that the corresponding lock can be reacquired by the locking thread.
- The global interpreter lock of the embedded scripting engine is not re-entrant.
- (programming) That may be executed more than once at a time either by different threads, or because of recursion.
- Global variables must not be altered by reentrant code.
- (transport, of a part of a rocket) Designed to return to the Earth's atmosphere.
- (physiology) Of or pertaining to reentry (in the heart)
Derived terms 
reentrant (plural reentrants)
- An angle or part that reenters itself.
- One who enters (the labour market, etc.) again.