- either of the pair of hind wings of an insect
- any of various noctuid moths (esp. genus Catocala) having brightly colored hind wings that are hidden under drab forewings except during flight
- One of a pair of hind wings of an insect, especially a moth or butterfly.
- Any of various noctuid moths chiefly of the genus Calocala, having drab forewings and brightly colored hind wings that are not visible when the moth is at rest. Also called underwing moth.
- The underside of a wing, as of a bird or airplane.