Tetraethyllead (common name tetraethyl lead), abbreviated TEL, is an organolead compound with the formula (CH₃CH₂)₄Pb. Once a common antiknock additive/octane booster in gasoline (petrol), TEL usage was largely discontinued because of the toxicity of lead and its deleterious effect on catalytic converters. It is still used as an additive in aviation fuel for piston engine-powered aircraft.