A data haven is a computer or network that holds data protected by both technical means (ie: encryption) as well as legal means -- typically, being located in a sovereign nation with no extradition laws, and with no or poorly enforced laws against the most common uses of data havens:

There is an understood rule amongst most of those who establish or favour data havens that they should not be used to facilitate spam, terrorism, or child pornography.

HavenCo (centralized) and Freenet (decentralized) are two modern-day data havens, with the former utilizing the Principality of Sealand, six miles off the coast of Britain.

This use of the term was coined by Bruce Sterling in his 1989 novel Islands in the Net, and further expanded upon by Neal Stephenson in Cryptonomicon.

See also: anonymity, pseudonymity, crypto-anarchism, Crypto-societies