A piston and cylinder from a steam engine
A cylinder in an internal combustion engine is the space a piston travels in. It gets its name from the shape it makes, a "cylinder". More than one can be arranged to make a cylinder block. On most air cooled engines, they have heat sink fins and are removable from the crank case. They can be removed and replaced(resleaved) from water cooled engines also, but not without some machining. The piston is the same size as the two bases of the cylinder (the circular and flat surfaces). The Duality of the cylinder is that it works as a seal with the piston rings and a bearing to the piston itself. Along with the piston rings, the cylinder condition is one of the main factors of how long an engine will last.
In the accompanying drawing, which depicts a cross-section of a steam engine cylinder, the bottom sliding part is the piston, and the top sliding part is a valve that directs steam into the two ends of the cylinder alternately.