A cylinder in an internal combustion engine or external combustion engine is the space within which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or 'block'. A cylinder block is typically cast from aluminum before precision features are machined into it. The cylinders are then lined with 'sleeves' of some harder metal (typically iron), which are pressed into each of the cylinder spaces. A cylinder block sits between the engine crankcase and the cylinder head, translating the reciprocating motion of the pistons into the rotating motion of the crankshaft via connecting rods attached to the pistons and crank.
A piston is sealed in each cylinder by a series of metal rings that fit around the circumference of the piston in machined grooves. These rings are made of some variety of spring steel which makes positive contact with the hard walls of the sleeve (this contact, and the resulting wear explains the necessity of some hard surface lining the cylinder).
A typical water-cooled engine will have a single row of cylinders in the engine block. V_engines use a cylinder bank at each leg of the V (a V6 using two banks of three cylinders, a V8 two banks of four, etc). Many other engine configurations exist.
Air-cooled engines generally use individual cases for each cylinder to facilitate cooling. inline motorcycle engines provide a notable exception to this norm, where two, three, four, or even six cylinder air-cooled motors use a common cylinder block to great effect. Water-cooled engines with only a few cylinders may also use individual cylinder cases, though this actually makes the cooling system more complex. The Ducati motorcycle company, for example, which for years used air-cooled motors with individual cylinder cases, left the basic design of their V-twin engine intact while adapting it to water-cooling.
A cylinder's displacement is defined as the area of the cylinder's cross-section (the bore) multiplied by the linear distance the piston travels within the cylinder (the stroke). This is called the 'swept volume' of a cylinder. The engine displacement is defined by the swept volume of a cylinder multiplied by the number of cylinders in the engine (all cylinders being of the same swept volume).
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.