The Collaborative International Dictionary
Cis- \Cis-\ A Latin preposition, sometimes used as a prefix in English words, and signifying on this side.
Douglas Harper's Etymology Dictionary
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" (cognates: 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 (such as cis-gender, by 2011).
pre. 1 On this side of. 2 (context chemistry English) (non-gloss definition: Forming names of chemical compounds in which two atoms or groups are situated on the same side of some plane of symmetry passing through the compound.) 3 (context gender English) cis#Adjective; cisgender or cissexual. 4 (context gender English) Of, related or specific to cis persons.