Normally I'd read "arc" as "atomic reference count", i.e., a refcount system where the increment and decrement operations are done with atomic increments/decrements. This permits the refcounted objects to be shared across threads that might mutate the refcount: they need to not corrupt the refcount, or UB ensues.

E.g., the Arc type in Rust. Rust, for example, has a separate Rc type. Rc should outperform Arc, but cannot be shared across threads.

You can manually increment and decrement reference counts with retain/release: https://stackoverflow.com/questions/6578/understanding-refer...

Like using std::shared_ptr<T> in C++ or Rc<T> (or Arc<T>, which in that case stands for atomic, not automatic) in Rust. That is, you as a programmer choose for which variables to use reference counting.