Is it possible to teach without a PhD?
Typically at larger "more prestigious" institutions, Professors must have their PhD to teach. It has been my experience that some smaller institutions will allow MS holders to teach undergraduate students - but your mileage may vary.

As an example, a friend of mine has currently taken over a Computer Science department at a small institution and only has his Masters. While they would prefer a PhD holder, they can not find one to take the job.

So, technically, yes - it is possible. Although you may not find yourself at a top-level institution.