Simple Definition:

Termination refers to the act of ending something or someone [bringing to a close]. It can refer to the conclusion of a contract, the dismissal of an employee from their job, or the end of a relationship. For example, when a company decides to terminate an employee, it means they are firing or letting go of that person.

Very Simple Definition:

Termination means to end something or someone. For example, when a company fires an employee, it means they are terminating their employment.