Dominant Strategy
Plain definition
A dominant strategy is a strategy that gives a player the highest payoff no matter what strategies the other players choose.
In economics
If every player has a dominant strategy, the game has a dominant-strategy equilibrium, which is always a Nash equilibrium. In the prisoner's dilemma both players have a dominant strategy to confess, which is why the inferior outcome is so stable.
Example
In an ad campaign game, if 'advertise' earns more profit than 'don't advertise' whether or not the rival advertises, then advertising is the dominant strategy.
Common confusion
A dominant strategy is not the same as the best possible outcome; it is only best relative to that player's own alternatives, and can lead everyone to a worse result.