1. the state of being active; the ability to produce some effect; the extent of some function or action. 2. a thermodynamic quantity that represents the effective concentration of a solute in a nonideal solution; if concentrations are replaced by activities, the equations for equilibrium constants, electrode potentials, osmotic pressure, boiling point elevation, freezing point depression, and vapour pressures of volatile solutes are converted from approximations that hold only for dilute solutions to exact equations that hold for all concentrations. The activity is equal to the product of the concentration and the activity coefficient, a dimensionless number measuring deviation from nonideality. Symbol a. 3. radioactivity; the number of disintegrations per unit time of a radioactive material, measured in curies or becquerels. Symbol A. 4. optical activity.