What is Schrödinger’s cat?

Austrian physicist Erwin Schrödinger imagined a cat sealed in a box with a radioactive atom, a Geiger counter and a vial of poison. If the atom decays, the counter triggers the release of poison and the cat dies. According to one interpretation of quantum mechanics, the atom is in a superposition of decayed and not decayed until observed, so the cat would be both alive and dead until the box is opened. Schrödinger meant it as a criticism, showing that this interpretation seemed absurd at human scale. It remains a popular way to discuss superposition, measurement and decoherence.