A single-acting cylinder in a reciprocating engine is a cylinder in which the working fluid acts on one side of the piston only. This is in contrast to a double-acting cylinder, in which the working fluid acts alternately in both directions. A single-acting cylinder relies on other cylinders, or the momentum of a flywheel, to push the piston back in the other direction.
Single-acting cylinders are found in both steam engines and internal combustion engines. They are also found in pumps and hydraulic cylinders.