Any change that a function makes to the program's state beyond returning a value, such as modifying global variables, writing to files, or printing to the console. Side effects make functions less predictable and harder to test.
Medical term from the 1880s describing unintended effects of medication, combining 'side' (Old English 'side') and 'effect' (Latin 'effectus'). Computing adopted the term in the 1960s to describe unintended or secondary consequences of function execution beyond the main return value.
Side effects are like a houseguest who not only eats your food (the main purpose) but also rearranges your furniture, changes your thermostat, and leaves dishes in the sink. The function does its job but also changes other things you might not expect!
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