Brownian Movement, or Brownian Motion, the rapid, erratic motion of microscopic particles dispersed in a liquid or gas. This motion is caused by the constant activity of the molecules in the fluid around the particles. These molecules collide with one another and with the dispersed particles. Individual particles observed through a microscope are seen to move at random in the fluid. Eventually, as a result of this random motion, all the particles are distributed throughout the fluid. When the fluid is heated, Brownian movement increases, because molecules move faster as the temperature rises.

Brownian movement is named for Robert Brown (1773-1858), the British botanist who first observed it.