Unicity distance
How much ciphertext you need before only one key yields readable plaintext.
Claude Shannon defined it in 1949 in Communication Theory of Secrecy Systems: the key's entropy divided by the language's redundancy per character. English carries roughly 3.2 bits of redundancy per letter. A simple substitution cipher has 26! keys, about 88 bits, giving a unicity distance of around 28 characters. Shorter cryptograms may have several plausible solutions; longer ones, in principle, only one.
The figure says nothing about how hard that solution is to find – only that it exists. For a one-time pad the distance is infinite, since the key is as long as the message and every plaintext of the right length is equally likely.