What is "underactuation"

Underactuation is a technical term used in robotics and control theory to describe mechanical systems that cannot be commanded to follow arbitrary trajectories in configuration space. This condition can occur for a number of reasons, the simplest of which is when the system has a lower number of actuators than degrees of freedom. In this case, the system is said to be trivially underactuated.

The class of underactuated mechanical systems is very rich and includes such diverse members as automobiles, airplanes, and even animals.