Where is the Frontier?

[Javert]

I was wondering, where is the Frontier?

Does it generally refer to any worlds on the edge of inhabited space, so it could be anywhere on the edge of the bubble of inhavited worlds? Or are there some particular systems that in lore are known as Frontier systems?

 

[pippo]

Space… the final frontier…
(you get the point)

Joke aside, on the edges of populated space.