If a track contains two channels, does that mean that the track is stereo?
Conversely, is a stereo sound always stored as a track with two channels recorded in the track?
Technically speaking stereo just means two or more sound sources. It is more related to how it is heard than represented. If you played a "stereo" track on one speaker it would be mono. Conversely if you played a mono track out of multiple speakers to make it sound like there is more than one source of audio "surrounding" the listener it would be stereo. The pleasantness of this works better if there is correlation between between the two but that's not required for the definition of stereo.
As an industry standard a "Stereo Track" is generally considered two channels on the same track or two channels each intended to come out of a different speaker. The words track and channel can get confused and interchanged depending on the medium however. Take a cassette tape for instance. It has four tracks physically. It plays two tracks at a time for the stereo effect. When you flip it over it plays two tracks from the other side of the tape. The reason we call a cassette stereo is because of its general use. On the other hand if you had a 24 track tape that's just 24 mono tracks that may or may not be used in a stereo way later when mixed down to a stereo environment.
Digitally it's simply an organization tool to make things easier when editing and mixing. In the same way you could have a multiple channels within a track that don't do stereo things so the label is changed to something like dual mono. It's just a tool that does mostly the same thing with a different label to help us organize. In general the "Stereo Tracks" are usually panned hard left and right for each track where the "Dual Mono" tracks are kept equal on both left and right outputs.