As an article from Law.com makes clear, you can call a defendant a lot of things during closing argument--but don't call him a terrorist. According to "'Inflammatory' Closing Leads to New Arson Trial," by Shannon P. Duffy, the closing argument of an Assistant U.S. Attorney, which took place the day before the first anniversary of 9/11, included these lines:
Standing here knowing what date today is I am very, very reluctant to use the term I'm going to use, but, frankly, I think this defendant warrants that term, and that term is terrorist.
There are many different kinds of terrorists. We all know too well the kinds of terrorists that caused the attacks of the anniversary so-to-speak we will mark tomorrow. But there are very different kinds of terrorists, and I think this defendant is one of them.
The defendant's lawyer didn't object, but the reviewing court held that the comment was prejudicial, and combined with other problems constituted "plain error." In ordering a new trial, the appellate court wrote: "We cannot know, given the evidence that came in, whether [the defendant] was convicted because the jury believed him to be an arsonist and the illegal possessor of a gun, or because it thought him to be a violent and dangerous man, a 'terrorist' of sorts."