Perfect secrecy
A cipher has perfect secrecy when the ciphertext reveals nothing at all about the plaintext.
Claude Shannon formalised the idea in 1949 in "Communication Theory of Secrecy Systems". The condition holds when the key is random, at least as long as the message, and used only once. The one-time pad is the only known scheme that achieves it, and its key material must never be reused.
The concept explains why practical encryption rests on computational hardness. Perfect secrecy is impractical for ordinary traffic, so modern ciphers accept that an attacker can try, but must fail within any realistic time.