Why is the ninth month called September?
word-forming element meaning "on the near side of, on this side," from Latin preposition cis "on this side" (in reference to place or time), related to citra (adv.) "on this side," from PIE *ki-s, from root *ko- "this" (cf. Old Church Slavonic si, Lithuanian šis, Hittite ki "this," Old English hider, Gothic hidre "hither;" see he). Opposed to trans- or ultra-. Originally only of place, sometimes 19c. of time; 21c. of life situations (e.g. cis-gender, by 2011).
Often cis- Having a pair of identical atoms or groups on the same side of a plane that passes through two carbon atoms linked by a double bond: cis-butene.