A collateral adjective is an adjective that is identified with a particular noun in meaning, but that is not derived from that noun. For example, the word bovine is considered the adjectival equivalent of the noun cow, but it is derived from a different word, which happens to be the Latin word for "cow". Similarly, lunar serves as an adjective to describe attributes of the Moon; moon comes from Old English mōna "moon" and lunar from Latin luna "moon". The adjective thermal and the noun heat have a similar semantic relationship. As in these examples, the collateral adjective very often derives from the Latin or Greek translation of the noun. In some cases both the noun and the adjective are borrowed, but from different languages, such as the noun air (from French) and the adjective aerial (from Latin). The term "collateral" refers to these two sides in the relationship.
Attributive usage of a collateral adjective is generally similar in meaning to attributive use of the corresponding noun. For example, lunar rocket and moon rocket are accepted as synonyms, as are thermal capacity and heat capacity. However, in other cases the two words may have lexicalized uses so that one cannot replace the other, as in nocturnal view and night view, or feline grace but cat food (not *cat grace or *feline food).
Collateral adjectives contrast with derived (denominal) adjectives. For the noun father, for example, there is a derived adjective fatherly in addition to the collateral adjective paternal. Similarly, for the noun rain there is derived rainy and collateral pluvial, and for child there are derived childish and childlike as well as collateral infantile and puerile.
The term "collateral adjective" was coined by the Funk and Wagnalls dictionaries, but as they are currently out of print, the term has become rare. A synonym sometimes seen in linguistics is suppletive (denominal) adjective, though this is a liberal and arguably incorrect use of the word 'suppletive'.
- Funk and Wagnalls Standard Desk Dictionary (1984) Harper and Row
- G. Jan Wilms, "Computerizing a Machine Readable Dictionary", in Proceedings of the 28th annual [ACM] Southeast regional conference, 1990
- Tetsuya Koshiishi, "Collateral adjectives, Latinate vocabulary, and English morphology", in Studia Anglica Posnaniensia, January 2002.
- "The typology of suppletion", a chapter in David Beck (2006) Aspects of the theory of morphology
- A Nocturnal View of the Lunar Landscape, Verbatim: The Language Quarterly