From the Comprehensive Rules (Rivals of Ixalan (January 19, 2018))
- 606. Loyalty Abilities
- 606.1. Some activated abilities are loyalty abilities, which are subject to special rules.
- 606.2. An activated ability with a loyalty symbol in its cost is a loyalty ability. Normally, only planeswalkers have loyalty abilities.
- 606.3. A player may activate a loyalty ability of a permanent he or she controls any time he or she has priority and the stack is empty during a main phase of his or her turn, but only if no player has previously activated a loyalty ability of that permanent that turn.
- 606.4. The cost to activate a loyalty ability of a permanent is to put on or remove from that permanent a certain number of loyalty counters, as shown by the loyalty symbol in the ability’s cost.
- 606.5. A loyalty ability with a negative loyalty cost can’t be activated unless the permanent has at least that many loyalty counters on it.
From the glossary of the Comprehensive Rules (Rivals of Ixalan (January 19, 2018))
- Loyalty Ability
- An activated ability with a loyalty symbol in its cost. See rule 606, “Loyalty Abilities.”