Torque:

Torque, also called moment or moment of force (see "Terminology" below), is the tendency of a force to rotate an object about an axis,

[1] fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist.
In more basic terms, torque measures how hard something is rotated. For example, imagine a wrench or spanner trying to twist a nut or bolt. The amount of "twist" (torque) depends on how long the wrench is, how hard you push down on it, and how well you are pushing it in the correct direction.
The terminology for this concept is not straightforward: In physics, it is usually called "torque", and in mechanical engineering, it is called "moment".

[2] However, in mechanical engineering, the term "torque" means something different,

[3] described below. In this article, the word "torque" is always used in the physics sense, synonymous with "moment" in engineering. The symbol for torque is typically t, the Greek letter tau. When it is called moment, it is commonly denoted M. The magnitude of torque depends on three quantities: First, the force applied; second, the length of the lever arm.

[4] connecting the axis to the point of force application and third, the angle between the two. In symbols:
where t is the torque vector and t is the magnitude of the torque, r is the lever arm vector (vector from the axis to the point of force application), and r is the length (or magnitude) of the lever arm vector, F is the force vector, and F is the magnitude of the force, × denotes the cross product, ? is the angle between the force vector and the lever arm vector. The length of the lever arm is particularly important; choosing this length appropriately lies behind the operation of levers, pulleys, gears, and most other simple machines involving a mechanical advantage.