Two friends who are neverseen without one another and always come to events or outings together as a pair. Often counted as a singular person in conversation. They are often clingy female friends or male relatives
People that you wouldn't normally hang out with, but since your on a field trip with them you become friends for the day.
Person 1: Hey how was that field trip yesterday?
Person 2: Oh it was fun me and John were field trip friends!
Person 1: Oh cool, I heard hes a fun field trip friend.