The Collaborative International Dictionary of English v.0.48:

Mercaptan \Mer*cap"tan\, n. [F., fr. NL. mercurius mercury + L.
   captans, p. pr. of captare to seize, v. intens. fr. capere.]
   (Chem.)
   Any one of series of compounds having an -SH radical
   attached to a carbon atom, also considered as hydrosulphides
   of alcohol radicals, in composition resembling the alcohols,
   but containing sulphur in place of oxygen, and hence called
   also the sulphur alcohols. In general, they are colorless
   liquids having a strong, repulsive, garlic odor. The name is
   specifically applied to ethyl mercaptan, C2H5SH. So called
   from its avidity for mercury, and other metals.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

ethyl mercaptan \eth"yl mer*cap"tan\, (Chem.)
   A gaseous substance (C2H5.SH) used as an odorant in natural
   gas and propane to facilitate detection and serve as a
   warning of leaks.
   [PJC]