Firstbits

Firstbits refers to abbreviating a Bitcoin address such that only enough of the initial characters of the address are given so that the full address can be identified from the block chain.

Any time a Bitcoin address is used for transactional purposes and appears in the block chain, its firstbits are considered assigned. If another address is created and used with the same prefix at a later time, the firstbits of that new address will be at least one character longer, as the shorter prefix goes to the address that was used first.

The firstbits of the very first address to be used by Satoshi in the genesis block of the block chain will have been "1".