Boundary/ˈbaʊndri/ Definition 1 of 4Pro Tip 1/3Physical Space Used for describing a visible, measurable separation between areas or objects. Make sure to paint within the boundary lines. Pro Tip 2/3Real-world Objects Often associated with tangible lines like fences, walls, or painted markings. The boundary wall separates our garden from the alley. Pro Tip 3/3Geographical Edges Refers to the edges of cities, states, and countries on a map. The river forms the natural boundary between the two countries.