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action, in general

Activity, an alternative name for the game charades

Activity, the ability of a piece to influence the game in chess

Activity, the rate of a catalytic reaction, such as enzyme activity, in physical chemistry and enzymology

activity (chemistry), the effective concentration of a solute for the purposes of mass action

activity (project management)

activity (radioactivity), the number of disintegrations per second

activity (software engineering)

activity (soil mechanics)

activity diagram, a diagram representing activities in UML

HMS Activity, an aircraft carrier of the Royal Navy