Orgeat (pronounced 'Ohr-Zsa' as in Zsa Zsa Gabor - with the 't' silent), is a syrup traditionally made from almonds, sugar and rose water or orange flower water. It is used as a powerful and flavourful sweetener and adds a distinctly almond/marzipan and bittersweet notes to drinks.
Historically, the recipe has included a barley blend ('Orge' is French for barley), and in the past has also been alcoholic, but nowadays it's typically non-alcoholic.
While orgeat has become conceptually tied to tiki (a thoroughly 20th century concept in terms of bartending culture), orgeat's use in mixed drinks goes back much further - to the late 19th century, in fact, with "Professor" Jerry Thomas using it in The Bon Vivant's Companion. Its first mention comes in the orgeat punch - a combination of orgeat, brandy, lemon juice and a dash of port - to which an accompanying footnote explains: (A decoction of which barley was formerly an important ingredient but which is now prepared with an emulsion of almonds). He also calls for it in the Japanese cocktail (where it is served alongside a good quantity of bitters, brandy and lemon peel) along with the Whiskey Daisy and a Temperance drink, Orgeat Lemonade.
But it's Trader Vic and his contemporaries who have become synonymous with orgeat - you'll see it used in the Fog Cutter, the Scorpion, Kava Bowl, Royal Hawaiian Cocktail and the Cocoanut Grove Cooler. Some modern classics that demand orgeat are emerging too, and not just from tiki bars - from PDT in New York's Beachbum to The Merchant Hotel in Belfast's Kon Tiki Ti-Punch.