Strategic actions that are "not part of the rules" but "that emerge naturally as the game is played". You can encourage emergent gameplay by adding more verbs, adding verbs that can act on many objects (ie guns that can shoot people and locks), goals that can be achieved in multiple ways, many subjects (ie characters, pieces on the board) and side effects that change constraints (ie moving a checkers piece limits the available moves for an opponent).
Related to Emergent Complexity and Meaningful Choices in Games
- [[The Art of Game Design - A Book of Lenses (2nd Edition)]] (Chapter 12. Some Elements Are Game Mechanics)