"In organic chemistry, a thiol is an organosulfur compound that contains a carbon-bonded sulfhydryl (–C–SH 
or R–SH) group (where R represents an alkane, alkene, or other carbon-containing group of atoms). Thiols are the sulfur analogue of alcohols (that is, sulfur takes the place of oxygen in the hydroxyl group of an alcohol), and the word is a portmanteau of "thio" + "alcohol," with the first word deriving from Greek θεῖον ("thion") = "sulfur". The –SH functional group itself is referred to as either a thiol group or a sulfhydryl group."
"Many thiols have strong odors resembling that of garlic, and indeed the odor of garlic itself is due to a thiol. Thiols are used as odorants to assist in the detection of natural gas (which in pure form is odorless), and the "smell of natural gas" is due to the smell of the thiol used as the odorant."
"Thiols are often referred to as mercaptans. The term mercaptan is derived from the Latin mercurium captans (capturing mercury) because the thiolate group bonds so strongly with mercury compounds."