MSH: An element that is in the alkali group of metals. It has an atomic symbol K, atomic number 19, and atomic weight 39.10. It is the chief cation in the intracellular fluid of muscle and other cells. Potassium ion is a strong electrolyte and it plays a significant role in the regulation of fluid volume and maintenance of the water-electrolyte balance.,CSP: metallic element of the alkali group, many of whose salts are used in medicine; atomic number 19, symbol K.,NCI: A metallic element that is important in body functions such as regulation of blood pressure and of water content in cells, transmission of nerve impulses, digestion, muscle contraction, and heartbeat.,NCI: An element with atomic symbol K, atomic number 19, and atomic weight 39.10.