A measurement is an interaction that crosses the Heisenberg cut separating the quantum from the classical world. Eg you prepare an atom in a superposition of st

A measurement is an interaction that crosses the Heisenberg cut separating the quantum from the classical world. Eg you prepare an atom in a superposition of states relative to some 'quantization axis' determined by a Stern-Gerlach apparatus. The state will randomly 'collapse' into one of the available final states, resulting in a macroscopic dot on some screen. You and all your physicist buddies will agree on the placement of the dot. There is no agreement on how this happens, complicated by the fact that there is no such thing as a classical world I alluded to above, ie any interactions with the apparatus and the conscious observers supposedly obey the laws of quantum mechanics.