Artemisia absinthium or Grande Wormwood is the active ingredient in real absinthe. Some copies use Southern Wormwood because of restrictions on thujone levels as it is said to be hallucinogenic. Interesting fact is that wormwood is used as a stuffing for goose..I also use it sparingly in sauces. There is a great recipe for chicken absinthe that I have somewhere...and it can also be used in fish sauces.
You need to hunt around to get the real stuff made in the European forests using fresh wormwood plants.
The best that I have ever found is
Century Absinthe which is a verte and has a dizzy thujone level of 100mg. As I have read the levels in old absinthe were about 260mg according to Dr Niels Arnold, although the market is now flooded with low thujone modern copies. There is a kind of absinthe which seems to be legal in the USA but most people who know import it from Europe.
Drink it with ice and sparingly
it is rather strong but has a lovely herbal flavour. Some of the other ingredients are anise, fennel, mint, calamus, hysoop, angelica and corriander. The simplest absinthes are blanches - they are clear white spirits that have not been steeped post distillation. As I say I prefer the European vertes (green) absinthe.