[GauntletWizard]

Atomic means "Does precisely what is is asked to do, or does nothing". This is the core of synchronization primitives, preventing two tasks from doing the same work or trying to access the same resource at once. open(O_CREAT| O_EXCL) for example, open a new file for you, or report an error. Regardless of who else is attempting operations at that moment, at most one of those calls will succeed (they can all fail, for a number of reasons). This allows you to, say, create a lockfile (often named filename.lock or .filename.lock) that serializes access to a shared resource, like a simple database or a printer control port or network connection. This is core to multithreaded programming (At least, imperative multithreaded programming; Functional languages tend to abstract this away by having little-or-no shared state)