▪ I. -ad, suffix1 of ns.
1. repr. Gr. -άδ-α (nom. -άς) forming, a. Collective numerals, as µονάς unity, monad, so dyad, triad, tetrad, pentad (especially used to class chemical elements or radicals according to the number of their combining units); hebdomad, chiliad, myriad, etc.; also perissad, Olympiad; decade retains final e from Fr. b. Feminine patronymics (in which it is a phonetic variant of -id), in proper names of females and districts, as Dryad, Naiad, Troad; often in pl. as Pleiad-es, Hyad-es, Cyclad-es. Hence c. in names of Poems, as Iliad, ‘the lay ({wlenisisub}δή) of Ilium,’ often imitated in modern times, as Lusiad, Dunciad, Rosciad, Columbiad; and d. used by Lindley to form family names of plants akin to a genus, as alismad, liliad, trilliad, asclepiad, etc. (on words in -a or after a vowel; otherwise -id, as in orchid).
2. a. Fr. ade-, in salad, ballad; see -ade the more usual form.
▪ II. -ad, suffix2
invented by J. Barclay in A New Anatomical Nomenclature, 1803, in the sense of ‘towards’ (the part denoted by the main element of the word), as caudad, cephalad, dextrad, dorsad, laterad, neurad.