one-time pad

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A key used in private-key cryptography that is the same length as the contents of the message being encrypted, providing unbreakable encryption if the key is perfectly random and used only once.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun cryptography A form of encryption where each bit or character from the plaintext is encrypted by a modular addition with a bit or character from a secret random key (or pad) of the same length as the plaintext.