I'm with JT on this one
To me a superstrat was a strat or similar double cutaway guitar that had either been hotrodded or was like that straight from the factory.
However the term wasn't really in common usage till the mid 80s onwards and by this time Charvel/Jackson, Kramer, Hamer, BC Rich, and a number of other companies were making them by the bucketload.
Ibanez were there to some extent with their Roadstar series but it wasnt till after they teamed up with Steve Vai in 87/88 that the whole Jem/RG thing happened (and the Jem was copied from Vai's Jackson soloists although given a few strat like features too. The RG body shape is a Jackson soloist shape with a deeper cutaway, just like Steve used to modify his)
To me it means one of these guitars that were a cut above a regular strat performance-wise. And stranger than that the only company whose product didn't get called that was Fender - partly because they were seen as traditional, & partly because of all the upheavals Fender went through during the mid 80s it meant that they came to the party late with their own versions.
These days when i hear it I assume that folks mean a generic Jackson, Charvel, Ibanez or similar guitar and itcan be neck through or bolt on, often with a Floyd or other locking trem and with a mix of Humbuckers and single-coils, but not necessarily.
It is a strat type guitar that can cover most musical styles - great if you are in a covers/function band or whatever.