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Derivational morphology forms lexical items from existing bases, changing meaning and sometimes lexical category. It is part of linguistic morphology, alongside inflection, which supplies grammatical forms of a lexeme. The distribution of an affix matters as well as its visible spelling.^[ref]

For example, adding -ness to the adjective kind gives the noun kindness; adding un- to kind gives another adjective, unkind. Combining the operations yields unkindness. A category-sensitive analysis first forms unkind, then attaches -ness: this use of un- selects an adjective, whereas kindness is already a noun. The intermediate structure explains a restriction that merely listing the three morphemes misses. Derivational morphology therefore connects reusable word-building rules to both syntax and semantics.

Inflection expresses grammatical distinctions through the forms of a lexeme, without normally creating a new lexical item. Grammatical number, grammatical tense and grammatical agreement can be expressed inflectionally, depending on the language. The distinctions and their overt expression must be kept separate: a grammatical meaning can instead be expressed by a separate word.^[ref]

In one boat and two boats, the ending marks a number contrast while both forms belong to the same lexeme. The pair walk and walked similarly contrasts grammatical forms of a verb. By comparison, deriving kindness from kind creates a new lexical item through derivational morphology. A computational description can store the base together with grammatical features, then predict its surface form. Irregular forms show why that prediction cannot always be reduced to concatenating one universal ending.

A lemma is a conventional citation form used to represent a lexical entry or a set of related grammatical forms. It differs from a lexeme, the abstract vocabulary item being represented, and from an arbitrary truncation of a word's spelling. The convention is language-specific.^[ref]

In a simple English analysis, walks, walked and walking can be assigned the lemma walk. The Universal Dependencies guidelines also distinguish inflection from derivational morphology: organizations has citation form organization, not organize or organ.

Context can be needed before the citation form is chosen. The spelling saw can name a tool or occur as a past form of see; a process that uses spelling alone cannot settle every such ambiguity. Recording the part of speech, intended word sense and the original occurrence makes a text corpus analysis more reproducible than retaining only the normalized label.

A lexeme is an abstract vocabulary item whose different grammatical forms are treated as instances of the same item. It separates lexical identity from the particular form occurring in an utterance. An inflectional description can therefore connect several forms without treating each as an unrelated entry.^[ref]

For example, walk, walks, walked and walking can belong to the lexeme conventionally represented as WALK. Their differences involve inflection; forming a new item through derivational morphology is a different relation. A lemma is the conventional citation form used to label an entry, rather than the entire abstraction being represented.

This distinction matters when counting a text corpus. Counts of the exact form walked answer a different question from counts pooling all the specified forms of WALK. The grouping procedure must also handle ambiguous forms: spelling alone need not determine which lexeme a use instantiates.

Linguistic morphology studies how words are structured from morphemes and how their forms relate to grammatical functions and meaning. It distinguishes reusable patterns from merely splitting a spelling into arbitrary pieces. Spoken and signed languages can both have morphological structure.^[ref]

For example, replayed can be analyzed as re- plus play plus -ed: repetition and past marking combine with a lexical base. The segmentation predicts relationships with play, replay and played, rather than only describing one isolated word. Inflection marks grammatical alternatives, while derivational morphology can form new lexical items. A computational analyzer can represent permitted combinations with a finite-state machine; an analysis is useful when it generalizes to new forms while rejecting combinations outside the specified grammar.