Abracadabra \ab-ruh-kuh-DAB-ruh\ , noun;
1. A mystical word or expression used in incantations, on amulets, etc., as a magical means of warding off misfortune, harm, or illness
2. Any charm or incantation using nonsensical or supposedly magical words
3. Meaningless talk; gibberish; nonsense
The etymology of this word is uncertain beyond the fact that it was borrowed from Latin in the late 1600's. One theory is that the word originated in Latin or Greek as an alteration of abecedarius ("abecedary*"). Another theory is that it is connected with Late Greek Abraxas (a name for the supreme god), but this link is tenuous at best. Other theories connect it to Thracian, Sumarian, Hebrew, Aramaic, and more.
Originally abracadabra was written out in a triangle shape on a charm or amulet and worn around the neck to ward off sickness, evil, etc.
*Check back for a definition and etymology of this in a couple days