Clarity in communication is vital. All my personal and professional experiences hammered home this point for my whole life. You must make sure that you are speaking from common ground when you communicate. Set the rules of vocabulary so that you both agree what you mean when you use those words.
Now my current understanding is this: clarity is 100% your job, and changing someone's mind about their own vocabulary is just about impossible. If someone is calling a kangaroo a tomato for some reason, it won't do you any good to point out that the real name is "kangaroo". You just have to go with it, and mentally file away a mental vocabulary mapping of "tomato = kangaroo" and remember to use it whenever that person is around. You don't always build common ground through discussing the vocabulary, but you can build common ground by swallowing someone else's vocabulary whole, and using that instead of what makes sense to you.
Mokalus of Borg
PS - Otherwise, you become that pedantic "actually..." interjector.
PPS - As I so often am, but I'm trying not to be.