Boundary definition
Learner's definition of BOUNDARY. 1. [count] : something (such as a river, a fence, or an imaginary line) that shows where an area ends and another area begins. Those two trees mark the boundary of our property. The river forms the country's western boundary.[+] more examples [-] hide examples [+] Example sentences [-] Hide examples.count] : something (such as a river, a fence, or an imaginary line) that shows where an area ends and another area begins. Those two trees mark the boundary of our property.