A piston ring is a split ring that fits into a groove on the outer diameter of a piston

Piston

A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...

 in a reciprocating engine

Reciprocating engine

A reciprocating engine, also often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert pressure into a rotating motion. This article describes the common features of all types...

 such as an internal combustion engine

Internal combustion engine

The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

 or steam engine

Steam engine

A steam engine is a heat engine that performs mechanical work using steam as its working fluid.Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be...

.

The three main functions of piston rings in reciprocating engines are:

1. Sealing the combustion/expansion chamber.
2. Supporting heat transfer from the piston to the cylinder
   Cylinder (engine)
   A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
   
    wall.
3. Regulating engine oil consumption.http://www.federalmogul.com/korihandbook/en/section_6.htm

The gap in the piston ring compresses to a few thousandths of an inch when inside the cylinder bore.
The split piston ring was invented by John Ramsbottom

John Ramsbottom (engineer)

John Ramsbottom was an English mechanical engineer who created many inventions for railways, including the piston ring, the Ramsbottom safety valve, the displacement lubricator, and the water trough.- Biography :...

 who reported the benefits to the Institution of Mechanical Engineers

Institution of Mechanical Engineers

The Institution of Mechanical Engineers is the British engineering society based in central London, representing mechanical engineering. It is licensed by the Engineering Council UK to assess candidates for inclusion on ECUK's Register of professional Engineers...

 in 1854.