Is buff a keyword or a limited action?

Buff is a Limited Action — not a keyword.

Under Rule 426, "Buff" is defined as an action (Buffing) that places a Buff counter on a unit. Rule 426.2 explicitly states: "Buffing is a Limited Action" — it can only be done when a game effect directs you to. It is not listed in the Keywords reference, nor is it a Permissive, Passive, Triggered, or Dependent keyword.

Buffs themselves are objects (counters) placed on units, each giving +1 Might (Rules 701–703), but the act of placing one is an action, not a keyword.