The rainbow; the iris.
[Via Latin from Greek iris, rainbow, iris.]
The Greek word could refer to the rainbow (Iris was the female messenger of the gods who appeared as the rainbow), to a type of lily, and to the coloured part of the eye surrounding the pupil. All these senses appear in various English compounds. Something iridescent shows luminous colours that seem to change when seen from different angles; the metallic element iridium was so named because it forms compounds of various colours; iridaceous refers to plants of the iris family (Iridaceae).
However, the larger number are medical terms referring to the iris of the eye, as in iridocyclitis (Greek kuklos, circle), inflammation of the iris and ciliary body of the eye; iridectomy (Greek ektomē, excision), a surgical procedure to remove part of the iris; iridodialysis (Greek dialusis, from dialuein, split, separate), a tear caused to the connection between the iris and the ciliary body as a result of an injury.