Moe's a slang term for an abstract concept, so there's no set definition.
In it's purest form I've understood it to be any trait that creates within an audience member a desire to protect or simply be around the character that isn't strictly sexual, so it differs from person to person. There are certain traits stereotypically considered moe such as big eyes, glasses, high pitched voices, naivety, clumsiness, poor health... but that's basically lowest common denominator stuff. You can stick a pair of big tits in there and a lot of people are gonna take not but not many are really going to go crazy for it.
Everyone is moe for different things. Personally, screw chiyo, Kagura is moe as hell.