Yes, the meanings of some terms are absolute according to Peter Unger's "Defense of Skepticism." Being absolute means that the term has a demanding positive claim, and therefore, it is not used to make true positive claims. For example, absolute words like "certain" also defines "knowledge," which possess demanding standards of application. As such, knowledge requires certainty. For example, "she knows that it is raining, but she is not actually certain of it," depicts some inconsistency in the assertion, and so people should be careful when using words with regards to whether or not knowledge requires certainty.