Wavefunction collapse

When a scientific experiment is done properly, it will give a measurable result. At each moment, the system (experiment) will be in one of several possible states. At the end, the experiment will be in a final state. At each point in time, the state of the system can be measured. 
Experiments done in quantum mechanics work the same way. The difference to classical mechanics is that at each point in time, several states are superposed (overlapping) to describe the state the experiment is in. These states are called eigenstates. In the same way as with classical mechanics, if a measurement is done, there is a single result. This result is the eigenvalue of one of the eigenstates. This means that measurement will reduce the several possible states to a single state by adding them together. After the measurement, the system will be in the state that was measured. In the Copenhagen interpretation, this reduction is known as wavefunction collapse. Collapse is one of two processes by which quantum systems evolve in time. The other is continuous evolution via the Schrödinger equation.
Werner Heisenberg was among the first to explain this situation, in a paper published in 1927. This result is controversial. Erwin Schrödinger used the thought experiment Schrödinger's cat to show this controversy.