Definition
Schrödinger's cat is a thought experiment with a strange conclusion: until you open the box and look, the cat has to be alive and dead at the same time. It was designed to show the contradiction that appears when you apply the physics of the tiny atomic world directly to the everyday world we live in.
What happens inside the closed box
Imagine a box with a cat and a special device inside. The device holds one radioactive atom and a bottle of poison gas. Over one hour, the atom has a 50% chance of decaying (giving off radiation and turning into a different atom).
If the atom decays, a detector senses it and drops a hammer, which smashes the bottle and releases the gas. If the atom stays intact, no gas is released and the cat stays safe.
In quantum mechanics, the physics of atoms, electrons, and other tiny things, an atom is treated as a 50-50 mix of "decayed" and "not decayed" until someone checks. So the cat, which is linked to that atom, would also have to be in a blend of states until you open the box: alive and dead at the same time. That's the strange conclusion.
To be more precise: why put a cat in a box?
To be more precise, the physicist Erwin Schrödinger, who proposed this thought experiment, was not celebrating how weird quantum mechanics is. He made up the story as a paradox to point out how absurd the explanations of the day sounded in plain common sense.
For invisible, tiny particles like electrons, "quantum superposition," where several possibilities are layered together, really does happen. But if you apply that same rule directly to a big, real-world object like a cat, you get a result that defies common sense.
In other words, Schrödinger used the extreme example of a cat to show how odd things get when you stretch the probabilistic rules of the tiny world out to everyday life. Of course, nobody ever put a living cat in a box and ran a dangerous experiment like this.
Does looking at it make reality "decide"?
So why can't we ever see a cat that is both alive and dead in daily life? In the past, some explained that the cat's fate gets settled only at the moment an observer opens the lid and looks inside.
Today, though, many physicists say that a person with a conscious mind isn't needed. Physical interaction with the surroundings is what breaks the overlapping state. The atom and the device inside the box constantly bump into the box walls, air molecules, light, and much more, trading energy all the time.
This quick collapse of a quantum overlap through contact with the environment is called decoherence (the loss of a superposition). In our large-scale world, it happens far faster than the blink of an eye, so we never see a cat in a blend of states. That said, physicists still disagree about whether decoherence alone explains why only one outcome out of many actually shows up.
Common misconceptions
Schrödinger actually put a cat in a box and ran a poison gas experiment.
It was a thought experiment that existed only in his head. No real animal was ever used. It was an analogy meant to expose a contradiction in the theory.
Schrödinger believed the cat really was half dead and half alive.
Quite the opposite. He created the paradox to show that the conclusion of a half-alive, half-dead cat is absurd, pointing to a problem in how quantum mechanics was being interpreted at the time.
Where you meet it
It's used as an analogy for a coin tossed in the air and covered with the back of your hand, where heads and tails both seem possible until you look. But a covered coin already has a fixed answer that you just don't know yet. In quantum superposition, the value itself isn't settled before measurement. That's the difference.
Quantum computers use superposition, where a bit is a blend of 0 and 1, to handle many possibilities at once. But the moment you read the result, only one answer is left. That's why they only get faster with special algorithms that combine waves so the probability of the answer you want gets larger.
In one sentence
It's an imaginary thought experiment that points out the contradiction you get when you drag quantum superposition from the tiny world into the world we live in.