That's a big question, especially if you are a young actor right out of college debating "Theatre Philosphy" at a bar.
Trying to appear smarter than me, he asked me "what is the most important element of theatre" to which I foolishly replied "imagery." (I was in my big design and symbolism phase at the time.) He told me "no," that "images were found in film: moving pictures" and that theatre was "about the words."
He was obviously so set in his way that I didn't want to get into the argument with him that literature was in fact about the words. Poetry, essays, and even radio are about the words. Theatre is not.
But what is theatre? I didn't answer because I couldn't come up with anything at the time.
After a few more years experience, I am happy to say that I have my answer. Theatre, at least to me, is about the live performance. The shared experience between artist and audience. The collaboration of the imaginations of all of the people in the same space at the same time.
But what do I know? What is your definition of theatre?