From Alex Bogomolny's March 2002 MAA Cut the Knot! column --

According to Steven Schwartzman, the word theorem originates with Greek theorema, theorein "to look at." Theater is a related borrowing, since "looking at" a play is what one does in a theater. And further: "A theorem was originally a sight or the act of seeing. Something that is looked at for any moment of time becomes an object of study. In mathematics, after studying a situation or a class of objects, a person hopes to make speculations and then prove them, so theorem came to mean the proof of a speculation that has been arrived at by looking at something." Theater, needless to say, came to mean something altogether different.

Mathematics — the only place where theorems live — and theater make an odd couple.