Activity may refer to—

• in chemistry, the effective concentration of an ion or other solute for the purposes of chemical reactions and other mass action. Activity differs from concentration, because of the affinity of solutes for the solvent molecules around them in solution. To some extent this affinity restrains solutes from engaging in other interactions (such as with reactants). Because their affinities differ, so do solutes' activity coefficients. These describe the constant of proportionality between a solute's concentration and its activity in a given solvent (typically assumed to be water). Compare: fugacity.
• in thermodynamics, a quantity that represents the concentration of a solute in a nonideal solution.
• in radioactivity, the number of disintegrations per second. It is measured in becquerel. For a given radioactive object, the activity is proportional to the number of atoms remaining. See: exponential decay.
• in project management, see activity (project management) .
• in chess, the ability of a piece to influence the game. An active piece controls more squares and has more moves available to it than an inactive piece.
• generally: action.