Encryption of sufficient strength so as to prevent your kid sister from cracking it. Includes everything (depending on the boredom of one's sister) from breakfast cereal decoder rings to homemade XOR encryption schemes budding programmers often think are really nifty before they read Applied Cryptography.

Modernly, DES, the American Data Encryption Standard, is starting to fall into this category...

(Also, under new US law (specifically the DMCA, or Digital Millennium Copyright Act), kid sister encryption is all that is necessary to provide an "effective access control", attempting to bring the mathematical art of cryptography into the murky waters of US law. (thanks yerricde.)