dērīvātĭo (noun F) : derivo
* A leading off, turning off, turning away.
* Lit.: derivationes fluminum, * Cic. Off. 2, 4, 14: sollemnis (sc. lacus Albani),Liv. 5, 15 (cf. shortly before, priusquam ex lacu Albano aqua emissa foret).
* Trop.
* In gen.: dictum aliquod in aliquem usum tuum opportuna derivatione convertere,Macr. S. 6, 1.
* Esp.
* In grammat. lang., derivation, etymology of words, Plin. ap. Serv. Aen. 9, 706; Dig. 50, 16, 57; Charis. p. 73 P. et saep.
* In rhetor.
* An exchanging of one word for another of like meaning, to soften the expression (as fortis for temerarius, liberalis for prodigus, etc.), Quint. 3, 7, 25.
* As rhet. fig. = παρηγμένον, the development of a preceding statement or conception into a new thought, Jul. Ruf. Schem. Lex. § 14.

s. TLL