Caesium is a chemical element, symbol Cs — a very soft, silvery alkali metal. You could cut it with a knife, and it melts not far above room temperature. It reacts so fiercely with air and water that you almost never meet the bare metal; labs keep it sealed, often under oil. Learner dictionaries mention photoelectric cells; the use that quietly runs the modern world is the caesium atomic clock, which is how the official second is defined. It is a specialist, science-class word, not an everyday one. British English writes caesium; American English writes cesium. Same element.