A dative covalent bond is a bond formed when one atom donates a lone pair
of electrons to the other: i.e. both atoms forming the covalent bond come
originally from the same atom.
See http://www.newi.ac.uk/buckleyc/bonding.htm (found just by searching
Alta Vista with "dative covalent bond").
I hope this helps.
Best wishes,
Dr. Clare Sansom
c.sansom@mail.cryst.bbk.ac.uk
Coordinating Tutor, Principles of Protein Structure