sigmoid

Borrowing from Ancient Greek σῑγμοειδής (sīgmoeidḗs, “of the shape of sigma”), from σῖγμᾰ (sîgmă, “sigma”) +‎ -ο- (-o-) +‎ -ειδής (-eidḗs, “-oid, -like”); Equivalent to sigma +‎ -oid. Compare with French sigmoïde.