Collapsed versioning is a strategy often used in pre-release branches that narrows the number of new versions generated by the commits in a branch so that they progressively end up to a single version which is the cumulative account of a whole range of commits. If you have already used pre-release branches you are probably confident with this kind of versioning but you probably wonder what's the algorithm behind.
Broadly speaking they are often used interchangeably but it depends on the perspective you're looking at it from. If you're asking is probably because in Nyx they are sometimes used as different entities.
The previous version is, broadly speaking, the version that came before the current one (current the one Nyx is in the process of releasing).
Nyx is named after 3908 Nyx, a notable Mars crossing asteroid, which in turn is named after the the Greek goddess of the night. We liked the name as it's three letters like the number of a semantic version number.