What if I see a vocabulary word on test day and I have no idea what it means? What should I do?

This is a great question, since it's likely that you'll encounter at least one question on the GRE where you're lost regarding the vocabulary.

First, Kevin wrote an excellent blog post that addresses this very question:

What to Do When You don't Know a Word on the GRE

To be honest, if you have absolutely no idea what any of the words choices mean, then your only real option is to guess. If you know what one of the words means, you might be able to eliminate at least one choice.

But even if you don't know what the word itself actually means, you might be able to use your intuition, rather than known definitions, to narrow down your options. This isn't a great technique, and should be a last resort, but let's go over your options:

1) Even though you don't know the exact definition of a word, you might be able to recognize the word, and have some sort of an understanding of how it's usually used. This could be as simple as thinking "I've heard this word used in negative situation before," or "I've heard this word used in this type of literature before." This kind of thinking can help you narrow things down a little bit.

2) You might be able to use word "roots" to guess the meaning of a word.

I would only use the two techniques above if you do not know the definitions of any of the words. These are the only real techniques beyond mere guessing that will be available to you — but they're not precise techniques.

All that said, the more words you know, the better off you'll be, clearly. So check out this question that deals with how many words you should learn for test day :)