costa

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A rib or a riblike part, such as the midrib of a leaf or a thickened anterior vein or margin of an insect's wing.

from The Century Dictionary.

• noun In Ctenophora, one of the eight meridional rows of swimming-plates.
• noun In Hydrozoa, one of the protective branches which form the walls of an open basketwork inclosing the gonangia, as in the corbula of Aglaophenia.
• noun The ridge of the sucker of a tapeworm.
• noun In anatomy: [L.] A rib.
• noun A border or side of something: specifically applied to the three borders or costæ of the human scapula or shoulder-blade—the superior or coracoid, the posterior or vertebral, and the anterior or axillary.
• noun A ridge on something, giving it a ribbed appearance.
• noun In zoöl.: In entomology: A broad, elevated longitudinal line or ridge on a surface. The anterior border of an insect's wing, extending from the base to the apex or outer angle. Hence— The space on the wing bordering the anterior margin. The costal or anterior vein.
• noun In conchology, the ridge or one of the ridges of a shell.
• noun In Actinozoa, an external vertical ridge marking the site of a septum within.
• noun In Crinoidea, a row of plates succeeding the inferior or basal portion of the cup.
• noun In botany, a rib or primary vein; a midrib or midnerve of a leaf or frond.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Anat.) A rib of an animal or a human being.
• noun (Bot.) A rib or vein of a leaf, especially the midrib.
• noun The anterior rib in the wing of an insect.
• noun One of the riblike longitudinal ridges on the exterior of many corals.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun anatomy A rib.
• noun biology A riblike part of a plant or animal, such as a middle rib of a leaf or a thickened vein or the margin of an insect wing.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any of the 12 pairs of curved arches of bone extending from the spine to or toward the sternum in humans (and similar bones in most vertebrates)
• noun a riblike part of a plant or animal (such as a middle rib of a leaf or a thickened vein of an insect wing)