TDC means Top Dead Centre. This is where the cycle of the four strokes starts in a car’s engine. The strokes are intake, compression, combustion and exhaust strokes.
1 Additional Answer
Top dead centre is the position of a piston in a reciprocating engine that is farthest from the crankshaft. Generally, the dead centre is any point of a crank where the applied force is straight along its axis; hence, no turning force is applicable.