The spice made from the seeds of the plant whose leaves are called cilantro is coriander.
Both coriander and cilantro come from the same annual herb, Coriandrum sativum, a member of the parsley family. In American English, “cilantro” usually means the fresh leaves and delicate stems, while “coriander” means the dried seeds used as a spice.
The two parts have noticeably different flavors. Cilantro is fresh, green, and citrusy, although some people perceive it as soapy because of genetic differences in smell-receptor sensitivity. Coriander seeds have warmer, nuttier, and lightly citrus-like notes and are often toasted or ground.
The terminology varies internationally. In British English and many other English-speaking regions, “coriander” can refer to both the leaves and seeds. Coriander is common in curry powders, garam masala, pickling blends, sausages, and gin botanicals.