Mechanical advantage definition

What is a mechanical advantage?

The ratio of the output force or torque to the input force or torque at any instant OR 
The ratio of force produced by a machine to the force applied to it is called the mechanical advantage of mechanism. 

For any linkage, if friction and inertia forces are ignored and the input torque is applied on one link and resisting torque is on another link then formula for finding mechanical advantage :

Power input = Power output 

 T1w1 = T2w2

So, Mechanical Advantage = T1/T2 = w1/ w2

Thus mechanical advantage is written as the reciprocal of the velocity ratio or the reciprocal of the torque ratio.