The Satoshi client or the Satoshi code refers to bitcoind, bitcoin-client, bitcoin-qt and Bitcoin Core. This is in honor of Satoshi Nakamoto for creating Bitcoin.

In the most widest sense:

All releases of bitcoin-x.y.z (starting 2009) and future official releases by the "bitcoin core developers".

In the narrow sense:

Only the bitcoin releases up to version 0.3.19 which Satoshi Nakamoto himself was chief and main developer. He retired afterwards (end of 2010) completely from this project. With the next release also Testnet was reset with a new genesis block.

Original Satoshi clients:

Only the bitcoin releases 0.1.0 up to 0.1.5 which supported only Windows 2000 / Windows NT and Windows XP (perhaps Windows Vista). The next bitcoin release 0.2.0 from Dec 2009, nearly a year later, starts to support Linux and the community got more and more actively involved in the development.

For history, one of the earliest available proof of a running bitcoin-0.1.0 with a human readable debug log. There might have been also (private) earlier client code before 0.1.0 available only to Satoshi Nakamoto.

See also