Field
Markets, information, risk, and human judgement — the theory of how people and prices actually behave.
Kahneman and Tversky showed that people systematically violate the 'rational' model of decision-making. Their prospect theory describes how we actually weigh gains, losses, and probabilities — laying the foundation for behavioural economics.
Black and Scholes derived a formula for the fair price of a stock option. By showing that an option can be perfectly hedged with the underlying stock, they removed the need to know investors' risk preferences and produced the famous Black–Scholes equation.
Akerlof showed how unequal information can wreck a market. Using the used-car market — where sellers know which cars are 'lemons' and buyers don't — he demonstrated that information asymmetry can drive good products out and even collapse trade entirely.
Mathematics
Nash's one-page note proved that every finite game with any number of players has at least one equilibrium — a set of strategies where no player can do better by unilaterally changing. The 'Nash equilibrium' became the central solution concept of game theory.