Caught Him Deleting Messages. Is He Cheating?

Cheating. What makes one a cheater? Does having an emotional affair count as cheating? What is one supposed to do if they suspect that their partner is having an affair? Is there such a thing as a reformed cheater?
Based on r/cheating_stories, featuring:
Caught him deleting messages
She has cheated in her last 2 relationships and vows not to cheat in our relationship, should believe her
My husband is texting a younger female coworker!