A bicycloalkane is a cycloalkane whose molecule contains two rings.  The number of rings in a cycloalkane is defined as the minimum number of bonds that must be broken in order to convert the molecule into an open-chain fragment.

eg:  Decalin

The minimum number of bonds that must be broken in order to convert Decalin into an open-chain fragment is two.  Decalin, therefore, is a bicycloalkane.

see also monocycloalkane, polycycloalkane