What is the boom on a c-arm oec 9800 in the OR?

I was using the c-arm today in the OR and the surgeon ask me to move the boom. in two years I have encountered many terms used to move the c-arm, never been told to move the boom. What is that? is that a technical term or just his terminology. I asked other x-ray tech around the dept. and they did not know what that was either.