Capstone/ˈkæpstəʊn/ Definition 1 of 3Pro Tip 1/2Architectural Significance Emphasizes the key role in stability and completion in construction. The ancient aqueduct stood solid, as its capstone was expertly placed. Pro Tip 2/2Symbolic Completion A metaphor for finishing a major task or a comprehensive effort. Completing the mural felt like putting the capstone on my years of study in art.