What defines a recumbent?
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.