tetraethyl lead

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

• noun A colorless, poisonous, oily liquid, C₈H₂₀Pb, comprised of four ethyl groups on each lead atom, used in gasoline for internal-combustion engines as an antiknock agent.

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

• noun chemistry a toxic organometallic compound, (C2H5)4Pb, added to petroleum/gasoline as an antiknock agent

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a clear oily poisonous liquid added to gasoline to prevent knocking