Boundary definition: a line or limit where one thing ends and another begins, or something that indicates such a line or limit.. See examples of BOUNDARY used in a sentence., A boundary is a border and it can be physical, such as a fence between two properties, or abstract, such as a moral boundary that society decides it is wrong to cross., The boundary of an area of land is an imaginary line that separates it from other areas., Definition of boundary noun in Oxford Advanced Learner's Dictionary. Meaning, pronunciation, picture, example sentences, grammar, usage notes, synonyms and more., Boundary, border, frontier share the sense of that which divides one entity or political unit from another. Boundary, in reference to a country, city, state, territory, or the like, most often designates a line on a map: boundaries are shown in red., Boundary refers to a line that marks the limits of an area, a dividing line. For example, the fence around a park marks the boundary of the park. The misspelling “boundry” is a common error. Always remember to spell it with an “a” after the “d” to ensure correct usage..