Stopover--when you stay more than four hours at a connecting point (24 on international flights--US to Canada or reverse is not international) even though there was a legal connection during that time.

Legal connection--there's a certain minimum time to catch the connecting flight. This varies by airline and airport. For instance, the minimum from United to Air Canada at O'Hare is 26 minutes. AC is well placed there, so that's not too bad, unless you're going to Winnipeg. Airlines generally will not sell tickets with connecting times below minimum; if you put one together yourself, it's at your own risk.

Circle trip--one that includes more than just two cities.

Open segment--a round trip in which you don't fly one segment. For instance, I might go CHA-ATL-LAX-ATL but leave off the final ATL-CHA.