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 new words (neologisms) by adding sounds (affixes) at the beginning (prefixation), the middle (infixation) or the end (suffixation) of words.

Positional categories of affixes

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.
In transcription, for example in the third column in the chart above, simple affixes such as prefixes and suffixes are shown connected to 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.

Lexical affixes

Lexical affixes (or semantic affixes) are bound elements that appear as affixes, but function as incorporated nouns within verbs and as elements of compound nouns. In other words, they are similar to word roots/stems in function but similar to affixes in form. Although similar to incorporated nouns, lexical affixes differ in that they never occur as freestanding nouns, i.e. they always appear as affixes.

Lexical affixes are relatively rare. The Wakashan, Salishan, and Chimakuan languages all have lexical suffixes — the presence of these is an areal feature of the Pacific Northwest of the North America.

The lexical suffixes of these languages often show little to no resemblance to free nouns with similar meanings. Compare the lexical suffixes and free nouns of Northern Straits Saanich written in the Saanich orthography and in Americanist notation:

Lexical suffixes when compared with free nouns often have a more generic or general meaning. For instance, one of these languages may have a lexical suffix that means water in a general sense, but it may not have any noun equivalent referring to water in general and instead have several nouns with a more specific meaning (such "saltwater", "whitewater", etc.). In other cases, the lexical suffixes have become grammaticalized to various degrees.

Some linguists have claimed that these lexical suffixes provide only adverbial or adjectival notions to verbs. Other linguists disagree arguing that they may additionally be syntactic arguments just as free nouns are and thus equating lexical suffixes with incorporated nouns. Gerdts (2003) gives examples of lexical suffixes in the Halkomelem language (the word order here is Verb Subject Object):