A dative covalent bond or a co-ordinate bond is a covalent bond in which the shared pair of electrons has been supplied by only one of the bonding atoms.
The shared pair of electrons is supplied by a lone pair on one atom.
The ammonium ion is formed when the lone pair on the nitrogen atom in NH3 is donated to a hydrogen ion H+ with an empty orbital.
Although shown with an arrow → all four bonds are equivalent and it is not possible to tell the difference.
