1. (noun)potassium, K, atomic number 19 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 Synonyms: chiliad, one thousand, kilobyte, honey oil, jet, kibibyte, superacid, atomic number 19, green, kelvin, super C, special K, kB, thou, grand, thousand, yard, cat valium, potassium