Define a dipole moment?

If you have two charges, #+q# and #–q#, separated by a distance #d#, the dipole moment #µ# is defined as the product of the charge and the distance between them.

#µ = qd#

In a polar molecule, there is a build-up of partial negative charge in some parts of the molecule and a corresponding partial positive charge in other areas.

In water, for example, the O atom has a partial negative charge (#δ^-# or #-q#).

The two H atoms each have a partial positive charge (#δ^+# or #+q#), with the centre of positive charge half-way between them.

This separation of charge constitutes a dipole.

In an electrical field, the molecule will experience a torque that causes it to rotate and line up with the field.

The positive charge will be attracted to the negative plate, and the negative charge will be attracted to the positive plate.

The amount of torque depends on the strength of the electric field and on the magnitude and separation of the electric charges.

If each charge has a magnitude #q# and the distance from the centre of the negative charge to that of the positive charge is #d#, the product #qd# is defined as the electric dipole moment #µ#.

#µ = qd#

In physics, a moment is the product of a physical quantity and a distance.

That's why we call it a dipole moment.