Dill seed is the fruit of the dill plant, while dill weed refers to the leaf and stem. 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 caraway, carrot, anise, celery, chervil, parsnip, parsley, fennel, and coriander. 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!

Dill weed is herb-like and dill seed is spice-like; the seeds have a stronger flavor than the weed. A fast easy way to use dill in your menus is to add fresh chopped dill to softened butter and use it as a spread on crusty bread or melt the dill butter as a dip for grilled seafood. Leftover tuna or egg salad makes a great appetizer or snack when put onto crackers and topped with a sprinkle of dill seeds. Fresh dill weed stores well in plastic bags in the freezer, while dill seed should be kept in a covered container in a cool dry place. Dried dill weed is less flavorful than fresh dill, but holds up better in dishes needing long cooking times.

Dill's history goes back to ancient times. Dill was documented as being used for medicinal purposes in India in 700 BC. The Vikings named the herb dilla, meaning soothing and used dill weed to help ease colic in babies. Today's research has found chemicals in dill called polyacetylenes, that may have cancer-fighting properties. Studies have shown that polyacetylenes slow the growth of tumors as well as help to prevent the growth of bacteria, mold, and yeast.

Although much maligned in French and Italian cooking, which favor fennel, dill weed and dill seed are widely used in Russian, German, Indian, and Scandinavian cooking. Dill seed and dill weed are used in distinctly different ways, although both may be used together in foods such as pickles, salad dressings, vinegars, and sauces. The use of dill weed in dishes such as potato salad is fairly common in the United States. Dill seeds are used in meat dishes in Indonesia and Malaysia, while dill weed is commonly used in rice dishes there. Dill seeds are used in condiments in Asia and in breads in Sweden.

The dill plant is bright yellow-green in color, with airy flowered heads and feathery spindly leaves. The heads, called umbels, look like upside-down umbrellas. Dill weed grows up to 3 feet (0.9144 meters) tall and imparts a lemony-pine essence with flavors reminiscent of family members caraway and fennel. Its fruit, referred to as dill seeds, are brown, flat, and oval-shaped. Dill seed contains more oil than dill weed and the seed oil is greasier and stronger tasting than dill weed oil.