[JackGibbs]

Perfect secrecy refers to the ability to determine any information about the plaintext without decoding it. Not having it can be very useful to an attacker, but that isn't always the case. RSA, for instance, doesn't have perfect secrecy, because it leaks the Jacobi symbol (https://en.wikipedia.org/wiki/Jacobi_symbol) of the plaintext. However, that information is of limited utility, and it can be shown that determining more useful facets, for instance the parity of the plaintext, requires solving more unfeasible problems.

[darkmighty]

Actually, perfect secrecy refers to the ability to determine any information about the plaintext at all, given arbitrary decoding power. It's quite simple -- it means that given a standard distribution of keys and an a priori distribution over the plain texts the best estimate of the plaintext given the ciphertext is simply the a priori distribution (no additional information). For the binary case, Y(any distribution)+X(uniform)=Z(uniform) (mod 2), so that this is satisfied for any prior.