Measure of the force applied to an object that causes it to rotate.
According to Wikipedia, torque is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist to an object.
Relationship between force F, torque τ, linear momentum p, and angular momentum L in a system which has rotation constrained in one plane only (forces and moments due to gravity and friction not considered).
Some people (e.g. American automotive engineers) use horsepower (imperial mechanical) for power, pound-feet (lbf⋅ft) for torque and RPM for rotational speed. This results in a formula for horsepower: