In linguistics, derivation is "Used to form new words, as with happi-ness and un-happy from happy, or determination from determine. A contrast is intended with the process of inflection, which uses another kind of affix in order to form variants of the same word, as with determine/determine-s/determin-ing/determin-ed.
A derivational suffix usually applies to words of one syntactic category and changes them into words of another syntactic category. For example, the English derivational suffix -ly changes adjectives into adverbs (slow → slowly).
Some examples of English derivational suffixes:
Although derivational affixes do not necessarily modify the syntactic category, they modify the meaning of the base. In many cases, derivational affixes change both the syntactic category and the meaning: modern → modernize ("to make modern"). The modification of meaning is sometimes predictable: Adjective + ness → the state of being (Adjective); (white→ whiteness).
A prefix (write → re-write; lord → over-lord) will rarely change syntactic category in English. The derivational prefix un- applies to adjectives (healthy → unhealthy), some verbs (do → undo), but rarely nouns. A few exceptions are the prefixes en- and be-. En- (em- before labials) is usually used as a transitive marker on verbs, but can also be applied to adjectives and nouns to form transitive verb: circle (verb) → encircle (verb); but rich (adj) → enrich (verb), large (adj) → enlarge (verb), rapture (noun) → enrapture (verb), slave (noun) → enslave(verb).
Note that derivational affixes are bound morphemes. In that, derivation differs from compounding, by which free morphemes are combined (lawsuit, Latin professor). It also differs from inflection in that inflection does not change a word's syntactic category and creates not new lexemes but new word forms (table → tables; open → opened).
Derivation may occur without any change of form, for example telephone (noun) and to telephone. This is known as conversion. Some linguists consider that when a word's syntactic category is changed without any change of form, a null morpheme is being affixed.