dipole

An electric dipole occurs when positive and negative charges (like a proton and an electron or a cation and an anion) are separate from each other. Usually, the charges are separated by a small distance.

A magnetic dipole is the closed circulation of an electric current system. A simple example is a single loop of wire with constant current through it. A bar magnet is an example of a magnet with a permanent magnetic dipole moment.

A dipole moment, \[\mu\], is a measurement of the separation of two opposite electrical charges. Dipole moments are a vector quantity. The magnitude is equal to the charge multiplied by the distance between the charges and the direction is from negative charge to positive charge. The magnitude of a single dipole moment is given by \[\mu=qd\], where \[q\] is the charge and \[d\] is the distance of separation.
For an array of charges (and if we also want to include the vector quantity), we use \[\overrightarrow{\mu}=\sum^{N}_{i=1}q_{i}\overrightarrow{d_{i}}\], where \[\overrightarrow{\mu}\] is the dipole moment vector, \[q_{i}\] is the magnitude of the \[i^{th}\] charge and \[\overrightarrow{d_{i}}\] is the vector representing the position of the \[i^{th}\] charge.

Sometimes the term "dipole" and "dipole moment" are used interchangeably, thus to avoid confusion, it's advised to used the term electric dipole moment, as per the recommendation of IUPAC.

A bond dipole moment, \[\mu\], is the dipole moment of a single bond, e.g. the bond between \[\ce{H-Cl}\]. It is defined as the product of the partial charge \[Q\] on the bonded atoms and the distance \[d\] between the partial charges, and it is given by \[\mu=Qd\] where \[Q\] is measured in coulombs and \[d\] in meters.

We can measure the partial charges on the atoms in a molecule using the equation above. If the bonding were purely ionic, an electron would be transferred between the atoms, and there would be a full +1 charge on the donor and a full -1 charge on the acceptor. Now, let's say we were to calculate the bond dipole between \[\ce{H-Cl}\], assuming that \[\mu=1.109\text{D}\approx3.7\times10^{-30}\,\text{C m}\], where \[\text{D}\] represents debye and the gas phase \[\ce{H-Cl}\] distance is \[127.5\,\text{pm}\], the charge of each atom would be \[Q=\frac{3.7\times10^{-30}\,\text{C m}}{127.5\times10^{-12}\,\text{m}}=2.90\times10^{-20}\,\text{C}\]. Knowing the charge, we can simply convert it in terms of electrons, \[\frac{2.9\times10^{-20}\,\text{C}}{1.6022\times10^{-19}\,\text{C}}\approx0.181\,e^{-}\].

This tells us that the \[\ce{H-Cl}\] bond has approximately 18 percent ionic character or 82 percent covalent character. However \[0.181\,e^{-}\] does not mean a literal potion of electron is transferred, but rather the electron density being slightly skewed towards the chlorine atom. We can therefore indicate the charge separation quantitatively as \[\ce{\overset{0.181\delta^{+}}{H} - \overset{0.181\delta^{-}}{Cl}}\].