A monocycloalkane is a cycloalkane whose molecule contains only one ring. The number of rings in a cyclic molecule is defined as the minimum number of bonds that must be broken in order to convert the molecule into an open-chain fragment.
The minimum number of bonds that must be broken in order to convert cyclopentane into an open-chain fragment is one, meaning the number of rings in cyclopentane is one. Cyclopentane, therefore, is a monocycloalkane.
Monocycloalkanes are by far the most common cycloalkanes. Thus, the term cycloalkane is commonly used to mean monocycloalkane.