WordNet (r) 3.0 (2006):

atomic number 19
    n 1: a light soft silver-white metallic element of the alkali
         metal group; oxidizes rapidly in air and reacts violently
         with water; is abundant in nature in combined forms
         occurring in sea water and in carnallite and kainite and
         sylvite [syn: potassium, K, atomic number 19]