The full form of bo id is Beneficial Owner Identification Number In simple words bo id is 16 digit unique identification number given to Beneficial owner at the time of Demat Account opening in CDSL through an authorized DP or Stock Broker. When you open a Demat account online, the CDSL will send you a  welcome letter with all of your account details along with bo id number and its also known as Demat account Number in case of NSDL.