Top dead centre (TDC) is when the piston in a reciprocating engine is at its highest point.
In a four-stroke engine, each piston reaches top dead centre twice during the engine cycle - once at the top of the compression stroke, and again at the top of the exhaust stroke.
Valve timing is often expressed as degrees before or after TDC (BTDC and ATDC respectively).
See also bottom dead centre (BDC).
