Stable Equilibrium
Stable Equilibrium is when a vertical line can be drawn from the object's centre of gravity directly through the base of the object it has stable equilibrium. If a hypothetical rider could have perfect balance and not be affected by any outside forces, they would remain in stable equilibrium and not fall over.
Unstable Equilibrium
The unicycle is an example of unstable equilibrium. In a perfect system it would remain perfectly balanced. But in the real world as soon as the centre of gravity is outside the base area, the force of gravity applies a torque about the contact point and pulls it to the ground.
For more see
Balancing.
