Source : Webster's Revised Unabridged Dictionary (1913)

Brownian \Brown"i*an\, a.
   Pertaining to Dr. Robert Brown, who first demonstrated (about
   1827) the commonness of the motion described below.

   {Brownian movement}, the peculiar, rapid, vibratory movement
      exhibited by the microscopic particles of substances when
      suspended in water or other fluids.