What is dBd and dBi and how do you convert from dBd to dBi?

dBd is a unit of measure for gain using a dipole as a reference; whereas, dBi uses an isotropic radiator.  To convert from dBd to dBi simply add 2.15 to the dBd value to get the dBi value (e.g. 0 dBd= 2.15 dBi)