Notifier reentrancy is an issue in asynchronous network programming on the MacOS (and possibly elsewhere). Your program interacts with the network through a notifier function that notifies you if certain things have happened -- for instance, if there is data waiting to be received.

A problem arises if you fail to handle notifications properly -- your notifier function could be called from within itself. Reentering the notifier in such a way tends to lead to infinite loops and other Very Bad Things.

Mostly, however, it's fun to say 'notifier reentrancy'.