Which term should I use when I want to buy a ticket from A to B for a scheduled bus, similar to what flight means for planes?
I agree with Drew's answer for American English: trip. Just as airlines tell their passengers to "enjoy your flight", the largest bus operator in America (Greyhound) tells its passengers to "enjoy your trip".
Having said that, you cannot use the words trip and journey for buses in all the same ways that you can use flight for planes. For example, you could say "My flight arrives at 8am", but you would never say "My trip/journey arrives at 8am". In that case you would just say "My bus arrives at 8am".
I have heard route used in this manner. Sometimes, the announcement is "Route north, Seattle to Vancouver, now boarding at gate #12."
I answered this question as parallel to Flight Number. As a teenager, I had the privilege to fly Pan Am One, the famous route beginning in San Francisco, going westward around the world, and ending in San Francisco.
Trains also use route, as Amtrack's Coast Starlighter. [Amtrack] : http://www.amtrak.com/coast-starlight-train
A flight is just one kind of trip, voyage, or journey. I don't know if English has some word specific to trips by bus, which is not applicable to other transport.
Trip. When you buy a bus ticket from A to B you buy a bus-trip ticket from A to B, that is, a ticket for the bus trip from A to B.
But you can just as well not use any word for the trip, if you are specifying A and B. Please give me a ticket from A to B is understood as asking for a ticket for the trip from A to B.
(And you might want to check out the site English Language Learners.)
When you are buying tickets, you can simply say bus ticket. (bus ticket vs. flight/airline/airplane ticket).
Technically, the equivalent term for flight is bus service for the scheduled bus trip sense which differs from a general bus trip. It shouldn't be confused with service that means an organized system for the needs of the public. I'm not saying that this term is what you use during a conversation or when buying a ticket, it is the equivalent technical term.
service - [countable] a bus, train, ship, or plane that goes regularly to a particular place or at a particular time
flight - an airplane scheduled to fly a certain route at a certain time
[Webster's New World College Dictionary]
In my opinion, bus trip/ride/journey/excursion is an equivalent of air/aircraft travel in general if there is no indication of a scheduled service. Though, these terms can be used within the right context and be equivalent to flight especially in the context of transport services. For example, there are trip planners or journey planners in the websites of these services.
Flight covers three meanings in this context:
a general trip/journey of an airplane
a scheduled trip of an airplane
an airplane making a scheduled trip.
As I mentioned earlier, the equivalent term for the scheduled trip sense is bus service. Though, the problem is that it is not very useful in everyday speech.
Most of the time, simply "bus" is used as an equivalent of flight in everyday speech. For example, you cannot say "My bus service is at 2pm". But you can say "My bus is at 2 pm".