Dill Seed, botanical name Anethum graveolens, is an annual herb in the celery family Apiaceae. Other members of this family include carrots and parsley. Dill seed is also known as American dill, dilly, European dill and seed dill. Dill Seed is not technically a seed, but the flat, oval, dark brown whole fruits of the herb. Dill Weed refers to the leaf and stem of the plant.
The dill plant is one of those rare plants that produces both an herb and a spice, others include Coriander, with the fruit known as Coriander Seeds and the leaves as Cilantro, and Fenugreek with the fruit known as Fenugreek Seeds and the leaves as Fenugreek Leaves. Although part of the same plant, dill seed and dill weed have such different flavor profiles that chefs would never substitute one for the other!