A Thing
A thing is when two people are secretly banging each other but they don't want to be called a couple. They just say they have a thing. It's like dating, but without the drama and the actual effort.
My mom and the mailman have a thing. They both know it. I just know they're not getting married.
My brother and his teacher have a thing. He still gets in trouble for it.
My friend and the school janitor have a thing. They don't even hide it anymore.