Word for "cinema with a single screen"?

Multiplex as a word has been around for centuries. It was not invented for cinemas.

As for the opposite, it seems to be uniplex - and uniplex, and uniplex.

Ngram shows 'uniplex' suddenly appearing in the 1970's, though it is only fair to point out that OED doesn't list the word.

Single screen itself is still used extensively in some countries, esp., where the multiplex (cinema) is a newer phenomenon and people are just learning the word. That way, it's also easier for them understand what is multiplex in the first place.

Though some may not be aware of it, uniplex is indeed used widely. However, what is to be noted is that when a newer technology/ system comes, we only need a distinguishing name for the newer one, the existing technology or system goes without a distinct name. Have you ever heard of analog TV? (it's what we always knew as TV before digital TV).

There is no generally-accepted retronym or back-formation yet for a non-multiplex movie theater.

These things take time; it was some 20 years after the touch-tone phone was introduced that we started seeing "dial telephone." The multi-plex is a little bit more recent than that, so any day now...

Personally, I say "big-screen theater."