Abracadabra is a magic word, used historically as a protective incantation on amulets and today mainly as the word a stage magician speaks at the climax of a trick.
The earliest known record of the word is in the Liber Medicinalis of the Roman physician Serenus Sammonicus, written in the second century AD. He advised people suffering from malaria to wear an amulet bearing the word written as a triangle, each line dropping a letter, on the belief that the shrinking word would funnel the sickness out of the body.
The word's origin is unknown. Proposed sources include an Aramaic phrase glossed as "I create like the word," a link to the Gnostic word abraxas, and a Hebrew phrase, but the Oxford English Dictionary notes that none of these conjectures has documentary support. By the early nineteenth century the word had passed from folk medicine into theatrical performance, and it now serves as the stock magic word of stage conjurors.