trivial name

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Chemistry A common, historic, or convenient name for a substance, derived often from the source in which the substance was discovered, but unsystematic and not used in modern official nomenclature, as sucrose for β-D-fructofuranosyl-α-D-glucopyranoside.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a commonly used, non-systematic name of a chemical compound. Trivial names for many compounds have been in use since long before their exact chemical structures were determined.