Quantum fluctuation refers to the temporary changes in energy that occur in empty space due to the uncertainty principle in quantum mechanics. This principle, formulated by Werner Heisenberg, suggests that certain pairs of physical properties, like position and momentum, cannot be precisely measured simultaneously. As a result, even a vacuum is never completely empty; it is filled with fleeting particles that pop in and out of existence. These fluctuations can lead to observable effects, such as the Casimir effect, where two closely placed plates in a vacuum experience an attractive force due to the energy of virtual particles. Quantum fluctuations play a crucial role in various phenomena, including the formation of cosmic structures in the universe.