monkshood

1. See aconite.
2. A slender, erect, poisonous perennial herb (Aconitum napellus) native to northern Europe, having violet flowers and whose dried leaves and roots yield aconite.

The noun has one meaning:

Meaning #1: a poisonous herb native to northern Europe having hooded blue-purple flowers; the dried leaves and roots yield aconite
  Synonyms: helmetflower, helmet flower, Aconitum napellus