What is coordinate bond?
A coordinate bond is a type of covalent bond formed between two species in which shared pair of electrons is denoted by one species only but shared by both species. It is also known as dative bond.
For example:
NH3 shares the pair of electrons present on Nitrogen with BF3, which is electron deficient.
Hence when in a bond between two species both the electrons are donated by one atom (unlike covalent bond in which both the atoms donate one electron each), the bond will be coordinate bond. The coordinate bond is denoted by an arrow from the donor atom of the electron donating species to the acceptor atom of the electron deficient species.