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1. The state of being active, the ability to produce some effect, the extent of some function or action.

2. (Science: chemistry) 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.

The potential or true thermodynamic activity of a substance, as opposed to its molar concentration.

3. (Science: radiobiology) The number of nuclear transitions or disintegrations occurring in a given quantity of radioactive material per unit time. The si unit of activity is s-1. The special name for the unit of activity is becquerel (bq). The previously used special unit of activity, curie (ci), is being replaced by the becquerel. 1 bq = 2.7 x E-1 3a5 1 ci. 1 ci = 3.7 x E10 bq.

4. Optical activity.

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