Can someone explain the use of “supersonic” in relation to space?

Great question! Made me search for answers. Although this answer does not have any sources, it seems to stand well on it's own:

Supersonic has a well-defined meaning. It means a speed that exceeds the local speed of propagation of pressure waves. In cold gasses, Mach 1 (the speed of sound) is rather low. In hot gasses, it is higher.

We don't talk about sound in space, because the density of gasses is too low to support any significant energy transfer to solid objects like eardrums. Nevertheless, "sound" waves (i.e. pressure waves) do travel through any gaseous medium. Therefore, the local speed of "sound" is well-defined.

By 'kathyastro': https://www.cloudynights.com/topic/815726-does-supersonic-have-any-meaning-in-space-nasa-press-release/