Emotional Intelligence
Emotional intelligence is the ability to perceive, understand, use and manage emotions in yourself and others. Peter Salovey and John Mayer defined the construct in 1990, and the phrase itself appears earlier, in Michael Beldoch in 1964 and Hanscarl Leuner in 1966. Daniel Goleman's 1995 bestseller is what made it famous, and it is not the origin. As a research construct it is narrower and duller than its reputation, which is why this page is marked mixed.
What it is
Two versions have to be kept apart, because almost every argument about emotional intelligence is really an argument about which one is being discussed.
Ability emotional intelligence is measured by performance tests, where a person is asked to do something with emotional information and is scored on how well they do it. That is the construct Salovey and Mayer defined in Imagination, Cognition and Personality in 1990. It is a modest, real ability that adds a little prediction of job performance beyond general intelligence and personality.
Trait emotional intelligence is measured by self-report: a questionnaire asking how emotionally skilled a person thinks they are. It overlaps heavily with existing personality traits, and a large part of what it measures already had names before it arrived.
The originator credit matters here more than usual, because the popular version of the construct and the research version diverge sharply and the popular version carries the famous name. Salovey and Mayer built it. Goleman sold it.