APA Dictionary of Psychology

a model of human development in which adulthood is divided into early, middle, and late segments, each period consisting of (a) transitional, or entry, stages (e.g., age-30 transition, age-40 transition), which are often times of uncertainty, self-examination, exploration, and modification of the quality and significance of life commitments, and (b) intervening periods of relative stability, when individuals consolidate new interpretations, structures, and goals and move forward. The adaptations associated with transitional periods may be relatively smooth and uneventful or may be experienced as psychologically difficult and painful (e.g., as a midlife crisis). [proposed by Daniel Levinson (1920–1994), U.S. psychologist]