A line that marks the limits of an area; a dividing line. The river forms the boundary between the two countries.
A limit of a subject or sphere of activity. The research pushed the boundaries of what was previously thought possible.
In cricket, a hit of the ball that reaches or crosses the edge of the field, scoring four or six runs. The batsman hit a boundary to secure the win for his team.
Mathematics: the set of points that marks the limit of a geometric figure or region. The boundary of a circle is its circumference.