The term ‘plasticity’ refers to the changeable character of matter. It is used in physics for inanimate materials and there it is contrasted with ‘elasticity’. If a coiled spring is pulled beyond the limits of elasticity, it will be permanently elongated. Provided that the spring does not break, the change is plastic. In the nineteenth century, the term was introduced into medicine to refer to the renewal of injured tissue and into popular literature to refer to impressionable minds. Plasticity was a dominant theme of James Mark Baldwin's (1902) book. It has returned in many other works about behaviour and the nervous system (e.g. Horn et al.,1973; Gollin, 1981; Lerner, 1984; Rauschecker and Marler, 1987).

Nowadays, plasticity, defined broadly in terms of malleability (see Pigliucci, 2001), is applied across a broad range of biological phenomena, and this extensive usage can cause confusion if the particular use is not well defined. Muscles that are not used diminish in size (atrophy) and those that are exercised become larger (hypertrophy). These are reversible phenomena. Many other cases occurring early in development usually are not. When one kidney fails to form, the other kidney undergoes compensatory hypertrophy and the outcome is stable. The behavioural repertoire of an individual can be changed by one of the many different types of learning, and at the molecular level the immune system responds to infection by developing a long-lasting reaction to the specific virus or parasite that caused the infection.