A cylinder is a solid shape bounded by a cylindrical shape and two parallel circular bases. Confused yet? Just picture a soda can or one of those mailing tubes with a round bottom and a lid.
The word cylinder is certainly mired in a whole lot of technical terms. It's one of the basic geometric shapes, as well as a key piece of engineering — such as the cylinders in an engine, which are the chambers where the pistons move. So don't worry if you don't quite get it. You have to be "running on all cylinders" to get what this word is all about.