The term 'outer boundary' can be defined by four different crossword solutions: PERIMETER (9 letters), LIMIT (5 letters), PERIPHERY (9 letters), and EDGE (4 letters). PERIMETER refers to the outermost boundary of a two-dimensional shape, such as a circle or square. LIMIT is a synonym for boundary, and is often used to refer to the boundaries of a physical space or area. PERIPHERY is another word for the outermost boundary of an area, and EDGE is a more general term for any boundary or LIMIT. All four words can be used to describe the outer boundary of a given area.