若一个进程或子进程可以「安全的被并行执行(Parallel computing)」，则称其为可重入（reentrant或re-entrant）的。即当该子进程正在运行时，可以再次进入并执行它（并行运行时，个别的运行结果，都符合设计时的预期）。可重入概念是在单线程操作系统的时代提出的。一个子进程的重入，可能由于自身原因，如执行了jmp或者call，类似于子进程的递归调用；或者由于硬件中断，UNIX系统的signal的处理，即子进程被中断处理进程或者signal处理进程调用。重入的子进程，按照后进先出线性序依次执行。

In computing, a computer program or subroutine is called reentrant if it can be interrupted in the middle of its execution and then safely called again ("re-entered") before its previous invocations complete execution. The interruption could be caused by an internal action such as a jump or call, or by an external action such as a hardware interrupt or signal. Once the reentered invocation completes, the previous invocations will resume correct execution.