The Stratum protocol is an open source standard protocol for a server and client relationship between a full node and a light client to synchronize Blockchain data. The protocol, which originated in Electrum, to service Electrum clients from Electrum servers was built upon by Slush in December of 2011 to coordinate mining for his mining pool. Although the protocol is in quite common use, it is not a BIP standard, its development was done out of band with the common Bitcoin effort.
Slush popularized and documented the protocol in 2012, as a full replacement for the dated getwork method. Unlike the less popular getblocktemplate and P2Pool pooled mining solutions, Stratum avoids the use of a validating full node, lightening resource requirements but also harming decentralization.