A [Satoshi][1] is the smallest fraction of a Bitcoin that can currently be sent: 0.00000001 BTC.  In the future, however, the protocol may be updated to allow further subdivisions should they be needed.

**Examples of units[2]**

 - 1 BTC = 1000 mBTC (millibitcoin)
 - 1 mBTC = 100,000 Satoshis   
 - 1 μBTC (microbitcoin) = 100 Satoshis

  [1]: https://en.bitcoin.it/wiki/FAQ#What_do_I_call_the_various_denominations_of_Bitcoins.3F
  [2]:
https://en.bitcoin.it/wiki/Units