Stated simply, Jack and Diane are in Nash equilibrium if Diane is making the best decision she can, taking into account Jack's decision, and Jack is making the best decision he can, taking into account Diane's decision. Likewise, a group of players are in Nash equilibrium if each one is making the best decision that he or she can, taking into account the decisions of the others. John Forbes Nash, Jr. is a mathematician who devised the principles of gaming theory and after which the Nash equilibrium is named. He also suffered from schizophrenia and was the subject of the movie, A Beautiful Mind. Nash married Alicia Lopez-Harrison de Lardé in 1957 but divorced in 1963. He was admitted to hospital for his mental illness and after his final hospital discharge in 1970, Nash lived in de Lardé's house. They remarried in 2001.
Find out more about the Nash equilibrium over at LinkedIn here.