n.1. a set of attributes that people have or are able to achieve relating to their ability to perform physical work and to carry out daily tasks with vigor and alertness, without undue fatigue, and with ample energy to enjoy leisure pursuits. 2. in biology, the extent to which an organism or population is able to produce viable offspring in a given environment or ecological niche, which is a measure of that organism’s or population’s adaptation to that environment. Compare unfitness. See Darwinian fitness; inclusive fitness; reproductive success. —fitadj.