Dill seed is the fruit of the dill plant, while dill weed refers to the leaf and stem of the dill plant. Dill seed is also known as seed dill and dill weed is sometimes referred to as leaf dill. Dill is in the family Apiaceae, which includes anise, caraway, carrot, celery, chervil, coriander, fennel, parsley and parsnip. The Apiaceae family has over 3,000 members, including the poisonous hemlock. Although part of the same plant, dill seed and dill weed are so different that expert cooks caution never to substitute one for the other!