Historical Rules · 1 February 2014
Rule 209.2
Official source text from the 1 February 2014 Rules release
An activated ability with a loyalty symbol in its cost is a loyalty ability. Loyalty abilities follow special rules: 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 none of that permanent’s loyalty abilities have been activated that turn. See rule 606, "Loyalty Abilities."