The Observer from the Rx pattern is the trio of callbacks that get subscribed to an Observable for receiving events.

The events received must follow the Rx grammar, which is: onNext * (onComplete | onError)?

That means an Observer can receive zero or multiple events, the stream ending either in one or zero onComplete or onError (just one, not both), and after onComplete or onError, a well behaved Observable implementation shouldn't send any more onNext events.