I always liked the way Bryan Cantrill defined it in one of his wonderful talks, which was basically that the difference between 'hard' real-time systems (eg missile guidance) and 'soft' real-time systems (eg whatsapp) boils down to the consequences of being late.

Traditionally, "realtime" means you can tell, in terms of actual wall clock time, how long different operations will take. Tends to show up in embedded systems because they tend to control devices in which fixed deadlines matter.

Somehow it morphed into a shorthand for "fast".

Yup, realtime was always predictable(ex. 16.666ms for 60fps). Time unit largely doesn't matter.

Generally precludes GC(yes there are edge cases where you can shoehorn GC in, I don't care about them since malloc is usually out of bounds as well).

If we want to talk about the minutia. None of these are "real time". Heck even the universe puts a limit on how quickly events can happen. We can look at the light from galaxies so far away that we see the galaxy billions of years into it's past. So nothing can be instantaneous.

However, I would say real time generally means acceptable latency or acceptable time reach to consistency. Although, I generally think of real time more along the lines of embedded systems.

Moreover, the word server-less really irks me. It's not server-less. You just don't have to manage the server or really interact with the server software that much. It's still a server.

server-lite?