Rule 700.1

Anything that happens in a game is an event. Multiple events may take place during the resolution of a spell or ability. The text of triggered abilities and replacement effects defines the event they’re looking for. One “happening” may be treated as a single event by one ability and as multiple events by another. Example: If an attacking creature is blocked by two creatures, this is one event for a triggered ability that reads “Whenever this creature becomes blocked” but two events for a triggered ability that reads “Whenever this creature becomes blocked by a creature.”