A Mathematical Theory of Communication Alive

Published in two parts in the Bell System Technical Journal (July and October 1948), Shannon's paper defined the bit as the unit of information, proved that any channel has a maximum capacity, and showed that coding can approach that capacity arbitrarily closely despite noise.

Before Shannon, 'information' was a metaphor; after, it was a measurable quantity. The theorems underpin every compression algorithm, modem, codec, and cryptographic analysis written since — and Shannon delivered them in prose so clean the paper is still assigned to undergraduates.