What is definition of Couple in engineering mechanics? Define moment of a couple?

The two equal and opposite forces, whose lines of action are different, form a couple, as shown in Fig. 1.4.

The perpendicular distance (x) between the lines of action of two equal and opposite forces is known as arm of the couple. The magnitude of the couple (i.e. moment of a couple) is the product of one of the forces and the arm of the couple. Mathematically,

Moment of a couple = P x X

A little consideration will show, that a couple does not produce any translatory motion (i.e. motion in a straight line), but a couple produces a motion of rotation of the body on which it acts.