Brownian is a Greek word which means “Leaping” also called “Pedesis”.
This kind of motion is first observed by a famous scientist Robert Brown in molecules of a substance.
Particle theory states that particle in a fluid or a gas has a random movement which is particularly in zig-zag fashion, this kind of motion in the molecules of matter is known as Brownian motion.
Fig: Brownian motion
Molecules moves randomly because the force in one direction is sometimes less than the force in other direction, so that the speed and direction of substance changes continuously and making zig-zag path.