From my Organic Chemistry days, I remember that the certain isomers (compounds with the same chemical formula but different structures), especially for *cis* and *trans* were italicized.

If you look on the Wikipedia page for [cis-trans isomerism](http://en.wikipedia.org/wiki/Cis%E2%80%93trans_isomerism), it seems to follow this convention, as every *cic* and every *trans* is italicized.

*Cis* is Latin for same side. A *cis* bond is one where the the substituent groups are both on the same side of the carbon-carbon double bond. Trans is Latin for *across*, meaning the groups are on opposite sides of the double bond.

I don't know why it started, but it's been done and repeated for so long that it is now conventional to do this.