The Brief: DTR stands for "define the relationship."

DEEP DIVE
DTR stands for “define the relationship.” It refers to a conversation that two people who are seeing each other (romantically and/or sexually) have so that they are on the same page about their relationship, feelings, exclusivity, etc.
Friends, colleagues, etc. can also DTR as friendly, professional, etc.
There are a bunch of new Facebook stickers and one says DTR and never would I have guessed that means Define the Relationship.
I'm not down with the hip new slang anymore
— Mitch (@mitchjaylineham) July 30, 2018
Hypothetical – when you start dating a person, do you have a discussion about exclusivity and deleting apps? Or do you prefer to delete when personally ready, and then DTR? (DTR = define the relationship)
— SuddenlySingle (@Single_CLE) May 12, 2018