Perimeter (noun)

1. The length of the boundary around a two-dimensional shape or object, especially a rectangle or circle.
2. The total length of the lines that enclose an area, such as a building, park, or military base.
3. The outer edge or boundary of a place, especially one that is under surveillance or control.
4. A secure or restricted area, especially one that is surrounded by a fence or wall.
5. A boundary or limit of a particular area or thing, especially one that defines the extent or scope of something.

Origin:

Late 19th century: from latin perimetrum, from greek peri- "around" + metron "measure".

Examples:

1. The perimeter of the square is equal to the sum of the lengths of all four sides.
2. The perimeter of the castle was fortified with a high stone wall to protect against enemy attacks.
3. The perimeter of the airport was on high alert following the security breach, with increased surveillance and security measures.
4. The perimeter of the nature reserve was marked by signs warning against unauthorized entry.
5. The perimeter of the city was expanding as more and more suburbs were being developed.