The Bank Verification Number, an eleven-digit number that identifies you across every bank in Nigeria. Your fingerprints are captured once and the number follows you between banks.
It is issued by the banking industry under the Central Bank of Nigeria, not by NIMC, and it is not your NIN. They are both eleven digits, which is the source of a great deal of confusion. You need both.
Dial *565*0# from your registered line to see yours.