OED entry for „caesium“. Second edition, 1989; online version June 2012. Retrieved 7 September 2012. Earlier version first published in New English Dictionary, 1888.
Caesium is the spelling recommended by the International Union of Pure and Applied Chemistry (IUPAC).[1] The American Chemical Society (ACS) has used the spelling cesium since 1921,[2][3] following Webster’s New International Dictionary. The element was named after the Latin word caesius, meaning „bluish grey“.[4] In medieval and early modern writings caesius was spelled with the ligatureæ as cæsius; hence, an alternative but now old-fashioned orthography is cæsium. More spelling explanation at ae/oe vs e.
