Definition
Plain language
A way of counting how many yes-or-no questions an attacker still needs to pin down a secret.
As stated in the literature
The residual Shannon entropy of the attacker's posterior over the secret; a drop of one bit means half the remaining possibilities have been eliminated.
Also called: bits of uncertainty, residual entropy
Why it matters: It gives a single number for how much closer an attacker has gotten, so partial leaks can be compared and tracked instead of dismissed as harmless.
For example, a four-digit PIN starts out worth about thirteen bits of uncertainty, and learning that its first digit is even chops off one of those bits.