Is there a clear technical definition for a recumbent? Is it the position of the knee in relation to the crank? The presence or absence of a back rest?
There seems to be a fine line between, for example, stretch cruisers and recumbents:
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Short answer: whoever is describing it.
There are a bunch of dictionary definitions around that all centre on the "lying down" part, which doesn't help when there are recumbents with quite upright seating positions:
as well as (semi?) recumbents with quite small backrests and seats off upright bikes:
At the other extreme, of course, you have the virtually horizontal positions mostly used for racing:
I am inclined to the view that anything with a laid back seating position and a backrest is probably a recumbent.
Note that the racing world has side-stepped the whole definitional question in favour of using "human powered" to describe what they care about. You get the "International Human Powered Vehicle Association" which sadly doesn't have a space vehicle category yet. They do have categories for number of riders, multitrack and single track, and age/sex ones. But no "recumbent vs prone vs upright" rider position ones.
A Recumbent is a bike with a SEAT, whereas other bikes are equipped with a SADDLE.
The difference here is that you can get out of a saddle while riding, for increased power or comfort or balance. You cannot get out of a seat while riding because its supporting all of your weight.
Horses have saddles because the rider can stand in the stirrups, whereas cars have seats because there's no way for the driver to effectively stand while driving. By that logic a motorbike seat should be called a saddle as well.
So a recumbent is any bike that has a seat instead of a saddle. The crossovers would be the semi-recumbent or crank-forward bikes which are a bit of both.