A short, casual way of saying "vocabulary" — the set of words a person knows, or the list of words you are studying. Students and teachers use it all the time: "today's vocab", "a vocab test", "I need to work on my vocab". It is not a formal word. You would say it to a classmate or write it in your notebook, but in a school report or an academic paper you would write the full word "vocabulary". It has no plural in normal use — people say "lots of vocab", not "lots of vocabs", because it behaves like a mass of stuff rather than countable things. Note that it usually points at the words themselves, so "a vocab list" and "my vocab is small" both work, but you do not use it for the study of words as a subject. One more thing worth saying: outside school, ordinary adults do use it too, mostly when talking about learning a language or joking about someone's fancy words.