Additament is a rarely used term that refers to something that is added to enhance or supplement another thing. Synonyms for additament include addition, supplement, appendage, adjunct, attachment, augmentation, extra, and add-on. These terms all share the basic meaning of adding something to another thing, but they may differ slightly in their nuances. For instance, an addition may be a larger or more significant addition than an add-on, while an adjunct may refer to something that is added to support or complement another thing without being essential to it. Regardless of the specific synonym used, all these terms convey the idea of something being added to enhance or complete another thing.