Could any native English speakers help me with some formal, informal, slang, or even some local (Scottish, Welsh, Irish, etc.) terms for "front passenger". I mean the term for a person that travels with somebody in a car (boat, aeroplane, train) next to the driver or even in the back-seat, etc.
A common slang term for riding next to the driver is "to ride shotgun". Apparently there is also the term "cobain" for the seat behind the shotgun seat but I've never encountered that one. Here's a source for both usages: http://www.urbandictionary.com/define.php?term=shotgun&defid=709676
"Shotgun" is by far the most common slang term for the passenger (or more precisely, the passenger spot) that's next to the driver/pilot. "Copilot" is another term I've heard a lot, and can refer to either a literal co-pilot in an airplane as well as the metaphorical "co-pilot" of a car or other vehicle.