I in a divorce and took in a housemate to help with utilities and my mortgage payments. She writes checks to these expenses and not to me. But my ex is claiming that it is rent and is making a claim on it. So I'm wondering what the legal definition of rent is. Thanks!

Webster says : 2 a: a usually fixed periodical return made by a tenant or occupant of property to the owner for the possession and use thereof ; especially : an agreed sum paid at fixed intervals by a tenant to the landlord b: the amount paid by a hirer of personal property to the owner for the use thereof.

You took on a tenant, a "housemate" as you say and she is handling a portion of the bills that you would otherwise have to pay - leaving you with more available money than you would otherwise have were you paying these expenses out of your own pocket. Whom it's paid to is irrelevant - you're benefiting (profiting) from the relief provided by having a "housemate" (tenant).

I hate to say it, but your ex has you on this one.