wormwood

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of several aromatic plants of the genus Artemisia, especially A. absinthium, native to Europe, yielding a bitter extract used in making absinthe and in flavoring certain wines.
2. n. Something harsh or embittering.

Century Dictionary and Cyclopedia

1. n. A. somewhat woody perennial herb, Artemisia Absinthium, native in Europe and Asiatic Russia, found in old gardens and by roadsides in North America. This plant is proverbial for its bitterness, and was in medicinal use among the ancients. It is of a highly tonic property, and is still used in Europe for weak digestion; it was formerly employed for intermittents and some other troubles, and was once regarded as a vermifuge. It is very largely consumed, with a few other species, in preparing the absinthe beverage of the French. (See absinthe and absinthium (with cut)). The name is extended to the genus, or particularly to species closely related to this; various species have their own names, as southernwood, mugwort, tarragon, santonica, and sage-brush.
2. n. Figuratively.
3. n. Bitterness.
4. n. By transference of the name, the common ragweed, Ambrosia artemisiæfolia, a bitter plant with foliage dissected somewhat like that of an artemisia.

Wiktionary

1. n. An intensely bitter herb (various plants in genus Artemisia) used in the production of absinthe and vermouth, and as a tonic.
2. n. Anything that causes bitterness or affliction.

GNU Webster's 1913

1. n. A composite plant (Artemisia Absinthium), having a bitter and slightly aromatic taste, formerly used as a tonic and a vermifuge, and to protect woolen garments from moths. It gives the peculiar flavor to the cordial called absinthe. The volatile oil is a narcotic poison. The term is often extended to other species of the same genus.
2. n. Anything very bitter or grievous; bitterness.

WordNet 3.0

1. n. any of several low composite herbs of the genera Artemisia or Seriphidium