A cylinder is a closed solid that has two parallel (usually circular) bases connected by a curved surface.
cylinder net via kwout
If you were to 'unroll' (make a flat net) the cylinder you would find that the side is actually a rectangle with a circle attached at each end.
Usually the bases are circles, so a familiar soup can would be technically called a 'right circular cylinder'. This is the most common kind, and if someone just says 'cylinder' this is usually what they mean.
The bases can however be almost any curved shape, but the most common alternative to a circle is an ellipse. The shape would then be called an 'elliptical cylinder'.
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