Nope, the term &quot;main thread&quot; is very explicitly defined by both iOS and OS X. Not only in all the frameworks (e.g. NSThread, NSRunLoop, etc. all explicitly define &quot;main thread&quot;), but even at the system level. OS X and iOS have a pthread function `int pthread_main_np(void);` that returns non-zero if the current thread is the main thread (meaning the initial thread that the application was launched with), and that is the thread that all UIKit (iOS UI) and AppKit (OS X UI) code must be running on.