Exhaust is an ability word, not a rules term of its own. It tags a family of activated abilities that share a shape: Exhaust — [cost]: [effect]. The oracle text then adds a reminder that each exhaust ability can be activated only once. Examples from the catalog include putting a double strike counter on a creature, adding three mana of one color, or giving a creature one or more +1/+1 counters. The label groups these one-shot activated effects together without changing how any of them work.
Because each exhaust ability is limited to a single activation, timing matters. You choose when to cash it in—before combat, in response to removal, or once the mana is available. The ability word helps players scan cards quickly and recognize that the text represents a finite resource rather than a repeatable activated ability.
