Priority Avenger

• WWizards of the Coast2019-11-12
  Official Ruling
  Non-instant spells with flash may still be cast any time a player has priority, regardless of what’s on the stack. The same is true of activating abilities.
• WWizards of the Coast2019-11-12
  Official Ruling
  Among abilities, only activated and triggered abilities use the stack. Activated abilities are written in the form “[Cost]: [Effect].” Triggered abilities use the word “when,” “whenever,” or “at.” They’re often written as “[Trigger condition], [effect].” Some keyword abilities are activated abilities and will have colons in their reminder texts, and some keyword abilities are triggered abilities and will have “when,” “whenever,” or “at” in their reminder texts.

Rulings via Scryfall