What is the definition of an action verb?

It is a word/verb that or which expresses an action done physically, or mentally, or either both, and it tells us what our subject of our clause (smallest grammatical unit that can express a complete proposition) or sentence is doing physically or mentally, or either both.

Examples of such Action are: Run, Jump, Kick, Break, Walk, Talk, Cry, Smile, Think, Eat, etc.

Because the function of an Action Word in a sentence is to describe what the subject itself is doing, either if it's a person, or something else, and an Action Word itself is also known as a "Doing Verb", because it includes words that tell what the subject itself is doing.