Absinthe is a potent, bitter liqueur distilled from wormwood and flavored with a variety of herbs, mainly anise. The Wormwood contains Thujone which is a mild hallucinogin.
Absinthe is usually diluted with water when it is served, which changes the color of the liqueur from green to milky white (like most anisettes).
Substitutes for Absinthe include Anisette, Herbsaint, Ouzo and Pernod.
Abuse of this liquer in the early part of the 1900s led to it being banned in the USA and France and a few other European contries. The only country that still makes it is The Czech Republic (Hills Absinthe).
You can make it yourself.
Warning: Because it's considered habit forming and hazardous to health, Absinthe is prohibited in most countries.