Lability refers to something that is constantly undergoing change or something that is likely to undergo change. Biochemically this is an important concept as far as kinetics is concerned in metalloproteins. This can allow for the rapid synthesis and degradation of substrates in biological systems. In medicine, the term "labile" means susceptible to alteration or destruction. For example, a heat-labile protein is one that can be changed or destroyed at high temperatures. The opposite of labile in this context is "stable."
Affective lability is also used as a psychological or medical term to describe lack of control over expression of emotion. For example, in people with the neurological disorder ALS, lability refers to uncontrollable laughing or crying brought on by neurological deterioration -- even though the patient may not find the situation particularly funny or sad.