在往复活塞式发动机中，止点（dead center)是指活塞能达到的距离曲轴最近或者最远的位置。前者称为下止点(bottom dead center, BDC)，后者称为上止点(top dead center, TDC)。

更广义地说，止点是使曲柄所受的力与其本身轴线共线的曲柄位置。也就是说，曲柄位于止点时，推动其转动的横向力为零。因此，一旦曲柄停在止点位置，它就无法靠机构本身来启动。

许多由曲柄驱动的机器（自行车、各类机械压力机、往复活塞式发动机等）利用飞轮储存一定动能，以顺利通过止点位置。多缸发动机则依靠合理设计来保证各个气缸的曲柄不可能同时位于止点。

In a reciprocating engine, the dead centre is the position of a piston in which it is farthest from, or nearest to, the crankshaft. The former is known as top dead centre (TDC) while the latter is known as bottom dead centre (BDC).

More generally, the dead centre is any position of a crank where the applied force is straight along its axis, meaning no turning force can be applied. Many sorts of machines are crank driven, including unicycles, bicycles, tricycles, various types of machine presses, gasoline engines, diesel engines, steam locomotives, and other steam engines. Crank-driven machines rely on the energy stored in a flywheel to overcome the dead centre, or are designed, in the case of multi-cylinder engines, so that dead centres can never exist on all cranks at the same time. A steam locomotive is an example of the latter, the connecting rods being arranged such that the dead centre for each cylinder occurs out of phase with the other one (or two) cylinders.