Semantic comes from the Greek word for "significant," and has to do with how, say, the word dog actually means that furry friend of yours, and all the others like him. If you're really into the philosophy of language and how words come to have particular meanings, then you like semantics. It can be an adjective, as in a semantic argument with your mom over the meaning of "grounded," or a noun, meaning "the study of signs and meaning."