- The maxim of quantum mechanics that both the position and velocity of a particle can never be definitely determined
- A principle stating that for a given particle it is not possible to simultaneously know with absolute accuracy both its location and momentum. No matter how accurate the measurements are, the mere observing or measuring said particle will interfere with it in an unpredictable fashion. This principle also applies to other observations or measurements. Also called Heisenberg uncertainty principle, or indeterminacy principle.