Side Effect
In plain terms
Any change a function makes to state outside its return value.
Going deeper
Side effects include modifying variables, writing files, or printing, anything observable beyond returning a value. Functional programming minimizes and isolates them.
Under the hood
Pure functions avoid side effects, easing testing and reasoning; effects make code harder to predict and parallelize.
Examples
console.log, writing to a database, mutating a global, network calls.
Think of it like...
Like a chef who, besides cooking your dish, also rearranges the whole kitchen.