Absinthe is a very strong, high-alcohol, herbal liqueur (spirit) with a licorice flavor. The drink had quite a cult following of famous writers and painters (think Hemingway and Picasso). The drink was so popular there were special fountains, glasses, and spoons manufactured for the sole purpose of serving the alcohol properly. The average alcohol content ranges from 45% to 75%.
In the early 1900s, the drink was banned in the U.S. supposedly due to a chemical compound called thujone, which was thought to drive people "mad." Most likely, it was just because the beverage was so high in alcohol. Finally, the ban was lifted in the early 1990s. Today you can still purchase the specialty serving tools as well the liquor. Even the old antique pieces can be bought on eBay. You can buy absinthe in most well-stock liquor stores. Expect to pay a minimum of $30 to $60 a bottle for anything "drinkable."
If you don't have absinthe, you can substitute one of these similar flavored beverages (per 2 tablespoons needed):