A fish on the Great Barrier Reef continually acquires new information from its environment—the location of food, the murkiness of the water, and the sounds of distant ships, to name a few examples. But only some of that information is meaningful, in that it actually helps the fish survive. In various disciplines, from biology to artificial intelligence, identifying such meaningful, or “semantic,” information is a key challenge. Yet a broadly applicable, fully formal definition of this kind of information has never been developed.