A mistress is a woman who is in a relatively long-term sexual and romantic relationship with a man who is married to a different woman.

What do you call a married woman who cheats on her husband?

A woman who cheats on her husband is an adulteress.

What do you call a male cheater?

Most commonly the betrayer is referred to as an adulterer, although cheater works as a more colloquial term. Im pretty sure it is gender neutral, as well. https://english.stackexchange.com/questions/12490/what-do-you-call-someone-who-betrays-his-her-spouse-sexually/12491#12491.

What is a good name for a cheater?

In this page you can discover 22 synonyms, antonyms, idiomatic expressions, and related words for cheater, like: swindler, crook, cheat, cozener, victimizer, deceiver, diddler, slicker, gypper, trickster and bilk.