An affix is a morpheme that is attached to a word stem to form a new word. Affixes may be derivational, like English -ness and pre-, or inflectional, like English plural -s and past tense -ed. They are bound morphemes by definition; prefixes and suffixes may be separable affixes. Affixation is, thus, the linguistic process speakers use to form different words by adding morphemes (affixes) at the beginning (prefixation), the middle (infixation) or the end (suffixation) of words.

Affixes are divided into several categories, depending on their position with reference to the stem. Prefix and suffix are extremely common terms. Infix and circumfix are less so, as they are not important in European languages. The other terms are uncommon.

Prefix and suffix may be subsumed under the term adfix in contrast to infix.

When marking text for interlinear glossing, as in the third column in the chart above, simple affixes such as prefixes and suffixes are separated from the stem with hyphens. Affixes which disrupt the stem, or which themselves are discontinuous, are often marked off with angle brackets. Reduplication is often shown with a tilde. Affixes which cannot be segmented are marked with a back slash.