1. In botany, the calyx is the sepals of a flower, taken as a group. The calyx is usually green in color and protects the flower when it is in bud. When the flower opens, the calyx surrounds the carpels, petals, and stamens, and forms the outer whorl of the perianth.
2. In anatomy, the calyx is a cup-shaped part, especially any of the divisions of the pelvis of the kidney. Each calyx receives urine from the urine-collecting tubes in one sector of the kidney. (Plural calyces.)