A 2000 bestselling mystery-thriller novel written by American author Dan Brown and published by Pocket Books. The novel introduces the character Robert Langdon, who is also the protagonist of Brown's subsequent 2003 novel, The Da Vinci Code, and 2009 novel, The Lost Symbol.

The book also contains ambigrams of the words Earth, Air, Fire, and Water, which has served to bring the art of ambigrams to public attention by virtue of the popularity of the book. The "Illuminati Diamond" mentioned in the book is an ambigram of the four elements arranged in the shape of a diamond.