Cypherpunk
In short: The cypherpunk movement promotes the use of cryptography and other privacy-oriented technologies as a way to achieve social and political progress.
The cypherpunk movement promotes the use of cryptography and other privacy-oriented technologies as a way to achieve social and political progress. A cypherpunk is someone who shares this view and, typically, builds tools that put it into practice.
Why it matters
The movement grew around a mailing list founded by Eric Hughes, Timothy May, and John Gilmore in the early years of the public internet, and its motto was cypherpunks write code. Its members argued that privacy should be protected by mathematics and freely available software rather than by promises from governments or companies. Many ideas behind cryptocurrency were developed or popularized in this community, including digital cash, pseudonymous identities, and Adam Back's Hashcash, a proof-of-work system cited in the Bitcoin white paper. Hal Finney, a well-known cypherpunk, received the first bitcoin transaction sent by Satoshi Nakamoto.
Example
Encrypting an email with PGP so that only the intended recipient can read it is a classic cypherpunk practice. The message stays private because of mathematics and freely available software that anyone can inspect, not because the email provider promises to keep it confidential.