Wiktionary
n. The ratio of the output force produced by a machine (especially a simple machine) to the applied input force.
WordNet
n. the ratio of the force exerted by a machine to the force applied to it
Wikipedia
Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. Ideally, the device preserves the input power and simply trades off forces against movement to obtain a desired amplification in the output force. The model for this is the law of the lever. Machine components designed to manage forces and movement in this way are called mechanisms. An ideal mechanism transmits power without adding to or subtracting from it. This means the ideal mechanism does not include a power source, is frictionless, and is constructed from rigid bodies that do not deflect or wear. The performance of a real system relative to this ideal is expressed in terms of efficiency factors that take into account friction, deformation and wear.
Usage examples of "mechanical advantage".
When steam was admitted to the cylinder, the short but immensely powerful stroke of the piston was turned by mechanical advantage into flailing speed at the far end of the arm.
Protrusions on the limbs where tendon rods connected gave it a huge mechanical advantage.
Now, how in hell can an apparently intelligent man know about rope and pulleys and not about mechanical advantage?
The mast was now so nearly vertical that shrouds and backstays were at a mechanical advantage, and everything could be hove taut regardless of the ship's motion.