Mutation
In plain terms
Changing the value of existing data in place.
Going deeper
Mutation modifies data after creation, which is efficient but can cause bugs when shared state changes unexpectedly. Functional programming favors immutability instead.
Under the hood
Shared mutable state is a top source of concurrency bugs; immutable data trades some performance for predictability.
Examples
list.append(), object property assignment, in-place array sorts.
Think of it like...
Like editing a document in place versus saving a new copy each time.