Wormwood is the main herb used to make absinthe. (It was also used in the original recipe for vermouth, which means "wormwood" in German.)

Wormwood is a wild bush native to Europe that also grows in North America. Oil extracted from the aromatic, bitter herb was originally used for medicinal purposes. But wormwood oil is believed to be poisonous and addictive in high doses and was demonized in the early 20th century when it was blamed for causing absinthe drinkers to go mad.

Absinthe was banned in the early 20th century, but never completely, and absinthe has come back into fashion in recent years.

Reference: What is absinthe? (Cookthink)
Reference: What is Mansinthe? (Cookthink)
Reference: What is vermouth? (Cookthink)