Claude Shannon — the American mathematician and electrical engineer who founded information theory. His 1948 paper "A Mathematical Theory of Communication" essentially created the framework for how we understand, transmit, and store information digitally. He also coined the term "bit" as the fundamental unit of information.
He worked at Bell Labs and MIT, and also did foundational work in cryptography and Boolean algebra as applied to digital circuits — showing that electrical switching circuits could implement logical operations, which underpins all modern computing.
A fascinating character personally too — known for riding a unicycle through Bell Labs hallways and juggling while doing so.
Best direct link:
That's a clean PDF hosted by Harvard — the full original paper. No login, no paywall.
There's also a version with LaTeX source at 9p.io/cm/ms/what/shannonday/paper.html if you want PostScript or gzipped formats.
It was originally published in two parts in the Bell System Technical Journal in July and October 1948, then republished as a book in 1949 with a preface by Warren Weaver — with the title tweaked to The Mathematical Theory of Communication, reflecting how fundamental it turned out to be.