Random deviations in the value of a quantity about some average value. In all systems described by quantum mechanics fluctuations, called quantum fluctuations, occur – even at the absolute zero of thermodynamic temperature as a result, ultimately, of the Heisenberg uncertainty principle. In any system above absolute zero, fluctuations, called thermal fluctuations, occur. It is necessary to take fluctuations into account to obtain a quantitative theory of phase transitions in three dimensions. The formation of structure in the early universe is thought to be a result of quantum fluctuations. Brownian movement is an example of a phenomenon that occurs because of fluctuations.