A co-terminal is two or more relatively adjacent airports which for the purposes of fare construction are considered the same point. i.e. LAX, BUR, LGB, ONT, SNA / SFO, OAK, SJC.

Usually roundtrip travel is defined as two each-way trips that depart from and return to the same co-terminal booked in a single reservation under one ticket number.

This works especially nicely when booking award travel if certain flights have availability and others do not. For example, you can book LAX-SYD-SFO-BUR and it counts as a round trip fare since BUR and LAX are co-terminals.