With the current value of 1 BTC exceeding 1000 USD the “Satoshi” as a unit has become more important to deal with. Here is the definition:

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

Further examples of units

1 BTC = 1000 mBTC (Millibitcoin)
1 BTC = 100,000 µBTC (Microbitcoin)
1 BTC = 100,000,000 Satoshis

1 mBTC = 100,000 Satoshis
1 µBTC = 100 Satoshis

In my opinion the current raise of the BTC value indicates that it’s impractical to use BTC as a currency. When you accept BTC you have do deal with the high volatile character of it’s value. Of course as a merchant you can adjust the BTC price tags based on your local currency (i.e. USD or EURO). But this doesn’t help much when you have them in the wallet: There is the risk of losing value – or the need to exchange them fast. So it may be a problem for merchants to accept BTC with it’s highly volatile character. Especially if you don’t sell digital goods but real world products. Yes, there are solutions out there to overcome these problems – but it makes look BTC a bit clumsy as a daily use currency.