Critical section
Also known as: Critical region
A critical section is the stretch of code that touches shared state and must not run concurrently with conflicting sections on that state.
You bracket it with a mutex or semaphore: acquire, touch the shared object, release. Keep it short. Long critical sections serialize threads and raise deadlock risk if you nest locks.
Without exclusion you get race conditions: results depend on interleaving. The Petri-net picture is enter/exit transitions around a resource token; the section is the path while the token is held.