Question

what is brownian motion?

Answer 1

The random motion of particles suspended in a fluid (liquid or gas) results from their bombardment by the fast moving atoms or molecules of the fluids (liquid or gas). 
This haphazard motion of the particles is known as "Brownian motion"..

Answer 2

the erratic random movement of microscopic particles in a fluid, as a result of continuous bombardment from molecules of the surrounding medium is called brownian motion.This transport phenomenon is named after the botanist Robert Brown.