In linguistics, an affix is a morpheme that is attached to a word stem to form a new word or word form. The main two categories are derivational and inflectional affixes. The first ones, such as -un, -ation, anti-, pre- etc, introduce a semantic change to the word they are attached to. The latter ones introduce a syntactic change, such as singular into plural, or present simple tense into present continuous or past tense by adding -ing, -ed to an English word. All of them are bound morphemes by definition; prefixes and suffixes may be separable affixes.