Probably everyone knows this,mbut just in case, Cis & Trans come from chemistry. Where is the "functional group" Cis= functional group, same side CH3
@gemswinc Chemistry got it from Latin, as in Gallia Cisalpina and Gallia Transalpina, the Gallic regions south of the Alps ("this side" to the Romans) and north of the Alps ("the other side").
Odd that some cis folk take exception to being identified as such.