A programming routine that can be used by multiple programs simultaneously. It is used in operating systems and other system software as well as in multithreading, where concurrent events are taking place. It is written so that none of its code is modifiable (no values are changed) and it does not keep track of anything. The calling programs keep track of their own progress (variables, flags, etc.), thus one copy of the reentrant routine can be shared by any number of users or processes.

Conceptually, it is as if several people were each baking a cake from a single copy of a recipe on the wall. Everyone looks at the master recipe, but keeps track of their own progress by jotting down the step they are at on their own scratchpad so they can pick up where they left off. The master recipe is never disturbed.