Dominant strategy
A dominant strategy is one that yields the best outcome for a player regardless of what others do.
Some decisions are easy: one option is best no matter what anyone else does. That is a dominant strategy, and where it exists, the game almost solves itself.
A dominant strategy is one that yields a better outcome for a player than any alternative, regardless of what the other players do. When a player has a dominant strategy, the choice is straightforward, since it is best in every circumstance, and predicting their behaviour becomes simple.
Best whatever happens
The defining feature is that a dominant strategy beats the alternatives across the board, against every possible combination of others' choices. The player need not predict what others will do, because the same action is optimal whatever they do. This is unusual and powerful: most strategic choices depend on anticipating others, but a dominant strategy cuts through that, removing the interdependence that makes games hard.
Dominant strategies and bad outcomes
The catch is that everyone following their dominant strategy can still lead to a poor collective result. In the prisoner's dilemma, betrayal is the dominant strategy for each player, best whatever the other does, yet when both follow it, both end up worse off than if they had cooperated. A dominant strategy guarantees a player does as well as possible for themselves given the structure, but it offers no guarantee that the resulting outcome is good for the group.
When they exist, and when they do not
Dominant strategies are the exception rather than the rule. Many games have none, so that the best choice genuinely depends on what others do, which is where concepts like the Nash equilibrium become necessary. When a dominant strategy does exist, it makes analysis easy and prediction reliable, which is why mechanism designers sometimes try to design systems, such as certain auctions, so that honesty or participation becomes a dominant strategy, removing the need for players to game the system.
A dominant strategy is the simplest situation in game theory: a choice that is best in all circumstances. Its value is in the clarity it brings, and its lesson, shared with the prisoner's dilemma, is that even when every player has an obvious best move, the outcome they collectively reach may be one none of them would have chosen.