Situated or intervening between successive ribs of the same side of the body: as, intercostal muscles, vessels, spaces.n. An intercostal structure, as an artery, and especially a muscle; an intercostalis.In iron ship-building, noting a structural member composed of a number of short pieces fitted in the spaces between a series of other continuous structural members which it crosses: as, an intercostal floorplate, one in which the floor is in short pieces between the longitudinals; an intercostal keelson, intercostal longitudinal, one in short pieces between the frames; an intercostal angle-bar, intercostal seam-strap, one in short pieces between frames or deck-beams, etc.