Max Born published a paper in 1924 entitled "Zur Quantummechanik", and this marked the first time that the phrase "Quantum Mechanics" was ever used. He hired Heisenberg and Pascaul Jordan to come and work for him on these problems and it was this fruitful triad that put Heisenberg's ideas into their most useful form.

But Born's most memorable contribution concerns the way in which we are to regard quantum mechanics. What is this wavefunction? What does it mean?

Born suggested that the only observable aspect of the wavefunction was its square, not the wavefunction itself. He held that the correct interpretation of the wavefunction was that the square at a given point in space was proportional to the probability of finding the particle at that point in space. The square is called the probability density while we can call the wavefunction is probability amplitude.