In my opinion the most important development in cryptography in the last 30 years is the precise definition of semantic security of a cipher. This definition captures the intuitive ideas of Martin Hellman, Ralph Merkle, and Whitfield Diffie of the mid-1970's tying cryptography to complexity theory that gave rise to the new cryptography with all its beautiful applications, multiple use of a key for file encryption, digital signatures, digital payment systems, one-way functions, one-way trapdoor functions, collision-resistant hashing, message digest codes, and so on.
My question is who came up with the precise definition of semantic security? Can you provide the reference to the paper that first defined semantic security precisely?