Room Separator

I believe it's called a threshold.

Do you mean between two different floor finishes, i.e. between tile and carpet, or wood and ceramics? If so, we (interior designers, architects, builders, etc.) call it a "reducer", although I'm not sure that's the correct term. It's meant to mask the different heights of two different floor finishes (so you don't trip), provide a transition between them and to protect the edge of the higher finish.

Oops, "threshold" is probably right, boomerang! (Although, we'd probably just use that term for exterior doorways.)