Definition of IRONWOOD. 1. : any of numerous trees and shrubs (as a hornbeam or hop hornbeam) with exceptionally tough or hard wood. 2.: the wood of an ironwood.See ironwood defined for kids.ADVERTISEMENT.. : any of numerous trees and shrubs (as a hornbeam or hop hornbeam) with exceptionally tough or hard wood. 2. : the wood of an ironwood. See ironwood defined for kids.