1966’s Babel-17 is an SF novel by Samuel R. Delany. Not his first (he had already published a number of Ace Doubles and one standalone), but the one that made his name. It shared the Nebula with Flowers for Algernon and was nominated for the Hugo as well, losing to The Moon is a Harsh Mistress. It shares some elements of its setting with an earlier Delany novel, Empire Star.
Victory over the Invaders may depend on understanding a series of indecipherable messages broadcast in an odd code? cipher? language? that the authorities label Babel-17. The Alliance turns to noted linguist Rydra Wong. “Tell us what this is and tell us what it means!”