A Hydraulic cylinder (also called a linear hydraulic motor) is a mechanical actuator that is used to give a linear force through a linear stroke.

Hydraulic cylinders are able to give pushing and pulling forces of millions of metric tons, with only a simple hydraulic system. Very simple hydraulic cylinders are used in presses; here the cylinder consists out of a volume in a piece of iron with a plunger pushed in it and sealed with a cover. By pumping hydraulic fluid in the volume, the plunger is pushed out with a force of plunger : area mulitiplyied by its pressure.

More sophisticated cylinders have a body with end cover, a piston-rod with piston and a cylinder-head. At one side the bottom is for instance connected to a single clevis, whereas at the other side, the piston rod also is foreseen with a single clevis. The cylinder shell normally has hydraulic connections at both sides. A connection at bottom side and one at cylinder head side. If oil is pushed under the piston, the piston-rod is pushed out and oil that was between the piston and the cylinder head is pushed back to the oil-tank again.