Page "Thread safety" Paragraph 15

; Re-entrancy: Writing code in such a way that it can be partially executed by a thread, reexecuted by the same thread or simultaneously executed by another thread and still correctly complete the original execution.

This requires the saving of state information in variables local to each execution, usually on a stack, instead of in static or global variables or other non-local state.

There are still some rare cases where non-local state can be used in a reentrant function, if the access is done through atomic operations and the data-structure is reentrant.