Particles suspending in a liquid or a gas perform irregular motion when they are bombarded by the liquid (or gas) molecules in random motion. This kind of motion is called Brownian motion. For example, smoke particles in air randomly bombarded by air molecules will have Brownian motion.

布朗运动是指在液体或气体中，悬浮粒子因为分子的随机运动而受到碰撞，进行的不规则运动。例如烟的粒子在空气中受到空气分子随机碰撞，便会产生这种无规则的运动。