What is the definition of a adverb?

An adverb is a word that describes an action

For example, if you said

The man ran fast.

Fast is the adverb because it's describing how her ran. Ran is the action. The word fast describes how the action is done.

a word or phrase that modifies or qualifies an adjective, verb, or other adverb or a word group, expressing a relation of place, time, circumstance, manner, cause, degree