From the Comprehensive Rules (Ixalan (September 29, 2017))
- 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 (Ixalan (September 29, 2017))
- Loyalty Ability
- An activated ability with a loyalty symbol in its cost. See rule 606, “Loyalty Abilities.”