Simple Definition:

Rhetoric is the art of using language effectively and persuasively [in a convincing way] to communicate and influence others. It involves the skillful use of words, techniques, and strategies to convey meaning and evoke [produce a strong feeling of] certain emotions or actions. For example, a politician may use rhetoric in their speech to convince people to vote for them.

Very Simple Definition:

Rhetoric is the skill of using words to make people believe or do something. For example, a teacher may use words to make students excited about learning.