Derivational morphology is a type of word formation that creates new lexemes, either by changing syntactic category or by adding substantial new meaning (or both) to a free or bound base. Derivation may be contrasted with inflection on the one hand or with compounding on the other. The distinctions between derivation and inflection and between derivation and compounding, however, are not always clear-cut. New words may be derived by a variety of formal means including affixation, reduplication, internal modification of various sorts, subtraction, and conversion. Affixation is best attested cross-linguistically, especially prefixation and suffixation. Reduplication is also widely found, with various internal changes like ablaut and root and pattern derivation less common. Derived words may fit into a number of semantic categories. For nouns, event and result, personal and participant, collective and abstract noun are frequent. For verbs, causative and applicative categories are well-attested, as are relational and qualitative derivations for adjectives. Languages frequently also have ways of deriving negatives, relational words, and evaluatives. Most languages have derivation of some sort, although there are languages that rely more heavily on compounding than on derivation to build their lexical stock. A number of topics have dominated the theoretical literature on derivation, including productivity (the extent to which new words can be created with a given affix or morphological process), the principles that determine the ordering of affixes, and the place of derivational morphology with respect to other components of the grammar. The study of derivation has also been important in a number of psycholinguistic debates concerning the perception and production of language.