• Reentrant is a term used in computing to describe a computer programme or subroutine that can safely run multiple invocations on multiple processors, or on a single processor system where a reentrant procedure can be interrupted in the middle of its execution and safely called again ('re-entered') before its previous invocations complete execution.
• Unlike recursion, where additional invocations can only be triggered by internal calls, the interruption could be caused by an internal action such as a jump or call, or an external action such as an interrupt or signal.
• This term comes from multiprogramming environments where numerous processes can run at the same time and control flow can be interrupted by an interrupt and transferred to an interrupt service routine (ISR) or 'handler' function.

Read More: What are the different states of a process?