One millibitcoin ("one Millie") is 0.001 BTC or 1 mBTC.
One microbitcoin ("one Mike") is 0.000001 BTC or 1 µBTC.
One satoshi is 0.00000001 BTC or one base unit (or 10 nBTC).
why not just deal in 'satoshi's? That will get rid of the need for mBTC/µBTC..
Psychology. If you have to spend 10,000 to get a pizza, it seems worthless.