In evolutionary biology:
In a nebulous sense, fitness is a measure of an organism's ultimate reproductive success. This is a composite of viability (the ability to survive), fecundity and the age of first reproduction. For a fundamental quantity of evolutionary biology, fitness is a difficult term to define to a high degree of rigour.
The first worthwhile elaboration to the general concept is that fitness should be measured at the same point in an organism's life cycle. Thus, producing more offspring than average does not guarantee high fitness, because it is possible that very few survive to maturity. A second elaboration that is somewhat less practical extends the concept of fitness forward in time. A genotype that is less successful than average may, owing to a change in the environment, gain a major advantage in later generations. The fitness of that genotype at an initial point in time is thus dependent on when its reproductive success is measured.
Though such elaborations are philosophically troublesome, this does not hinder the practical use of the term. In this sense it is much like the concept of velocity, of which quantity is dependent on the point of reference. For a better discussion of the meaning of fitness, see the relevant chapter in Keywords in Evolutionary Biology, eds. EF Keller and EA Lloyd. ©1992, Harvard University Press.
See also: evolution, adaptive landscape.