Interface
In plain terms
A contract specifying methods a class must implement.
Going deeper
An interface defines a set of method signatures without implementation, so any class implementing it guarantees those behaviors. It's a promise of capability.
Under the hood
Enables polymorphism and decoupling; a class can implement many interfaces.
Examples
Java interfaces, contracts, polymorphism, decoupling, API boundaries.
Think of it like...
Like a job description listing required skills any qualified candidate must have.