I have typed a similar message before, but I will include it again here.
Cis/trans are Latin terms, that are most usually associated with organic chemistry.
Cis means same, trans means across. There was Cisalpine Gaul and Transalpine Gaul, mentioned in the 3rd and 4th centuries BC, to mean (in reference to Rome) "this side of the Alps Gaul" and "on the opposite aide of the Alps Gaul".
For organic chemistry, it refers to a form of isomerisation (same atoms, different 3D arrangement)
In these isomers, there is a double bond, which the chemical groups cannot rotate around. They are locked in place.
A chemical with two groups on the same side is called "cis-name of chemical". A chemical with the groups on the opposite side of the double bond is called "trans-name of chemical"
This is because they have different properties. From memory, one example has the trans isomer which does nothing, whereas the cis isomer is a chemotherapy drug.
I will include an image in my next message.