John Forbes Nash (1928-2015) was an American mathematician that was Professor at Princeton University. In 1994 he received the Nobel Prize in Economic Sciences along with John Harsanyi and Reinhard Selten, for their works regarding game theory.
During the 50s (in papers originated after his PhD dissertation “Non-Cooperative Games”, 1950) he developed the so-called Nash Equilibrium that occurs when no player would change its strategy knowing the preferences and strategies of the other players. No player has anything to gain by changing only their own strategy unilaterally. This is, you would make your own best decision taking into account the other players’ decision.
His ideas and theories are not only used in economics and mathematics, but also in other very different fields such as politics and military theory.