A spice plant with aromatic seeds (Nigella sativa).
A cultivated variety of cumin (Cuminum cyminum).
A spice plant related to cumin with dark seeds and an edible root (Bunium bulbocastanum).
The strong-flavored seed of black caraway or nigella, Nigella sativa, which is deep black with flat sides, like those of the onion.
Used other than figuratively or idiomatically: A seed produced by an onion plant.