What is Theatre?

In my opinion theatre is any performance with an audience. This doesn't have to be in a conventional theatre space and instead it can be anywhere that there are spectators.
It can be a piece of performance ranging from naturalistic to experimental, physical to purely voice, maybe even only the use of sound or in fact all of it combined together.

I do however feel the performance must have a purpose, be it to entertain or inform. Ideally it must make people think as it is a great means of expressing hard hitting/taboo subjects to a large audiences and as an actor, it is our duty to do this. However on the occasion it can be nice to watch a piece of light hearted entertainment as it can be a source of escape for audience members, although this can be achieved through work that is informative. 
On the whole I think theatre is way for audiences to escape regular life and immerse themselves reality before them. It is a greats way to expand people's thinking, not only being a pleasant sight but also helping both the actors and audience to become more opened minded to different topics. 

In some so-called traditional methods of acting I do feel that some of theatre's purpose is lost. With actors wanting to get their performances right they remove their abilities to respond truthfully and instinctively, therefore removing the audience will to suspend their beliefs and a topic which maybe crucial to progress. 
An audience member, to me, must be willing to suspend their belief in order to connect with the performance and for this to occur the actors must be able to create an environment which allows them to do so. I've seen performances with thousands of pounds of set and I've seen others with no set at, of which both let me fully engage with the piece. This suggests to me that a performance is not about the best most accurate set, lighting or music, or in fact having any of these things at all, instead theatre focuses on the actors and what they can do to make an audience invest in the performance.

To me theatre is what actors can do to an audience to make them feel.