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.)