The noun perimeter refers to the border of an enclosed space. The perimeter of a house is its property line. For a country, it would be that country’s borders.
From Latin, meaning “around” (peri)and “measure” (metron), a perimeter is basically a boundary of any kind, measuring around the shape. In mathematics, perimeter refers to the length of this boundary. In geometry class, you might be asked to calculate the perimeter of a polygon, which is the sum of the length of each side. You might also hear this word used by members of the military or a police force as in “secure the perimeter” or “the perimeter has been breached.” A basketball player that likes to shoot from the three point line is said to “operate on the perimeter.”