A code path that can execute in parallel and that works on (reads and/or writes) shared writeable data (shared state) is called a critical section. They require protection from parallelism. Identifying and protecting critical sections from simultaneous execution is an implicit requirement for correct software that you – the designer/architect/developer – must handle.
A critical section is a piece of code that must run either exclusively; that is, alone (serialized), or atomically; that is, indivisibly, to completion, without interruption.
By exclusively, we're implying that at any given point in time, one thread is running the code of the critical section; this is obviously required for data safety reasons.
This notion also brings up the important concept of atomicity: a single atomic operation is one that is indivisible. On any modern processor, two operations are considered to always be atomic; that is...
