I've always heard of this kind of truck being called a cherry picker, since that was one of the most common things it was used for, but I guess "boom truck" makes sense as well. They would call it that because of the arm.
There's a few things that are also called "boom" like the boom mic that they often use on movie sets in order to capture sound without putting the mic in the picture. They also have a long, extendable arm.
And I know that the spar at the bottom of a sail is also called a boom.