Bottom dead centre (BDC) is when the piston in a reciprocating engine is at its lowest point.

In a four-stroke engine, each piston reaches bottom dead centre twice during the engine cycle - once at the start of the compression stroke, and again at the start of the exhaust stroke.

Valve timing is often expressed as degrees before or after BDC (BBDC and ABDC respectively).

See also top dead centre (TDC).