Superposition is the idea that a quantum system can hold two or more states simultaneously rather than being in just one. An electron can be in a superposition of spin up and spin down, and a photon can be in a superposition of two different paths through an experiment.
This is not the same as simply not knowing which state it is in. The system genuinely occupies both possibilities at once, and interference experiments prove it, since the two states can combine and cancel each other out in ways that only make sense if both were really present.
Superposition collapses to a single definite outcome the moment it is measured. It is the foundation of quantum computing, where a qubit in superposition can represent both 0 and 1 at the same time, letting certain calculations explore many possibilities in parallel.
The clearest demonstration of superposition is the double-slit experiment. Fire individual particles, one at a time, at a barrier with two narrow slits, and instead of building up two simple bands behind each slit, they gradually form a striped interference pattern, exactly the kind produced when two overlapping waves reinforce each other in some places and cancel out in others. This happens even when particles are sent through one at a time, implying each one is somehow interfering with itself, passing through both slits in superposition simultaneously rather than choosing just one. Place a detector at the slits to check which path each particle actually took, and the interference pattern vanishes instantly, replaced by two ordinary bands, since measuring the path collapses the superposition before it can interfere with itself.
Superposition is also the working principle behind some of the most sensitive measurement devices ever built. Atomic clocks and quantum sensors exploit superposition states to measure time, gravity, and magnetic fields with a precision that would be impossible using any purely classical technique, turning what once looked like an abstract quantum curiosity into some of the most practically useful physics in existence.