Princeton's WordNet(0.00 / 0 votes)Rate this definition: 1  costa(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) 2  rib, costa(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)