Torque is also known as moment of force or couple. When a force act on a particle does not merely move in the direction of the force, but it also turns about some point.
So, we can define the torque for a particle about a point as the vector product of position vector of the point where the force acts and with the force itself.
It is measured as the product of the magnitude of the force and the perpendicular distance between the line of action of the force and the axis of rotation.
The torque (τ) on the particle relative to the fixed point O is a vector quantity and it is defined as-