Historical Rules · 25 January 2019
Rule 605.1a
Official source text from the 25 January 2019 Rules release
An activated ability is a mana ability if it meets all of the following criteria: it doesn't require a target (see rule 114.6), it could add mana to a player's mana pool when it resolves, and it's not a loyalty ability. (See rule 606, "Loyalty Abilities.")