In linguistics, the modification of root words (lexemes) to create new words. Unlike inflectional morphology, the meaning and the part of speech may change.
English has few inflections but a significant number of derivational morphemes. Probably the most common derivational morpheme is the zero derivation, where a word changes meaning depending on context and not on any lexical change. An example is flood (the noun) and flood (the verb). This is the basis, by the way, for verbing.