  Symplectic mappings are discrete-time analogs of Hamiltonian systems. They
appear in many areas of physics, including, for example, accelerators, plasma,
and fluids. Integrable mappings, a subclass of symplectic mappings, are
equivalent to a Twist map, with a rotation number, constant along the phase
trajectory. In this letter, we propose a succinct expression to determine the
rotation number and present two examples. Similar to the period of the bounded
motion in Hamiltonian systems, the rotation number is the most fundamental
property of integrable maps and it provides a way to analyze the phase-space
dynamics.
