the
constant, random, zigzag movements of small particles
dispersed in a
fluid medium, caused by collision with
molecules of the
fluid. First observed by Scottish botanist
Dr. Robert
Brown (1773–1858) in examination, under a microscope, of liquids containing very small particles which were just large enough to be visible.
—Webster's New World Dictionary, Second College Edition and Collier's Encyclopedia (5th ACC Glossary Part 1) Final approval 2.12.89