Someone you're not in a relationship with but you act like you are. Often times the guy prefers video games and watching netflix over the girl and makes her feel unwanted. Can be a relationship status in itself without the extra baggage of saying girlfriend or boyfriend.
Susy: We really like each other but he hasn't asked me out. He probably never will so I just call him my datemate, and it rhymes!
Going somewhere or doing something that would traditionally be considered a romantic or 'date-like' venue or event with someone who is a friend and you have no plans to be romantically involved with. Traditionally used to describe a female-male pairing
Tom and I had an awesomematedate, we went ice skating, went for dinner and had a stroll along the beach