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.