Entry 1 sense Webster, 1913 Flagstone /flăg'-stōnˌ/ · Flag·stone · IPA /ˈflæɡˌstoʊn/ 01 n. A flat stone used in paving, or any rock which will split into such stones. See Flag, a stone. 1. A flat stone used in paving, or any rock which will split into such stones. See Flag, a stone. See: Flag