"The process whereby a (biological) system responds to its environment so as to adjust to the constraints of the environment and to take greater advantage of the environmental circumstances" (T.F.H. ALLEN & T.B. STARR, 1982, p.261)
Actually, the authors have defined "adaptation". in this way. However, "adaptation" would better describe some state at some instant. The global result of an adaptive process is a sequence of adaptive states, while the adaptive process mirrors the adaptive capacity of the system, i.e. its adaptability.
ALLEN & STARR coined their definition in relation to biological systems, but systems made of many living systems, especially human ones, are also adaptive.