Arsenic is a chemical element — one of the basic stuffs the world is made of — and its symbol on the chart is As. In everyday English, though, almost nobody is thinking of a square on a chart. They mean a famous poison. For a long time a white powder made from arsenic was used to kill people quietly: it has little taste, and the sickness can look like a bad stomach, which is why old crime stories are full of it. That everyday sense is still the one you will hear in films and news. Chemists, though, treat it as a real substance with many forms, not a magic powder. It also turns up in quieter, sadder ways: some well water and some old factory land have too much arsenic in them, and drinking or living with that for years can slowly harm a body. People sometimes say “arsenic” for any sneaky poison; that is loose talk. Cyanide is a different chemical. The word itself feels scientific and a little old-fashioned, but the poison story is known to almost everyone.