Activity may mean:

• Action (philosophy), in general
• The Aristotelian concept of energeia, Latinized as actus
• Physical exercise
• Activity (UML), a major task in Unified Modeling Language
• Activity diagram, a diagram representing activities in Unified Modeling Language
• Activity, an alternative name for the game charades
• Activity, the rate of catalytic activity, such as enzyme activity (enzyme assay), in physical chemistry and enzymology
• Activity (chemistry), the effective concentration of a solute for the purposes of mass action
• Activity (project management)
• Activity (radioactivity), radioactive decay#Radioactive decay rates, the number of radioactive decays per second
• Activity (software engineering)
• Activity (soil mechanics)
• HMS Activity (D94), an aircraft carrier of the Royal Navy
• in military parlance, a military agency or unit (e.g. Intelligence Support Activity)