an imaginary limit or border between one place or thing and another:
He was convicted of illegally importing weapons across state lines; a district/ county line; lines of longitude and latitude
— see also COASTLINE, DATELINE, DIVIDING LINE, PICKET LINE, TREELINE, WATERLINE
SYNONYMS border boundary • line • frontier
These are all words for a line that marks the edge of something and separates it from other areas or things.
border the line that separates two countries or areas; the land near this line: a national park on the border between Kenya and Tanzania boundary a line that marks the edges of an area of land and separates it from other areas: The fence marked the boundary between my property and hers. line an imaginary boundary between one area of land and another; a line on a map that shows this: lines of longitude and latitude frontier (British English) the line that separates two countries or areas; the land near this line: The river formed the frontier between the land of the Saxons and that of the Danes.
WHICH WORD? The point where you cross from one country to another is usually called the border. In British English it can also be called the frontier, but this is often in a context of wildness, danger and uncertainty: The rebels control the frontier and the surrounding area. The line on a map that shows the border of a country can be called the boundary but ‘boundary’ is not used when you cross from one country to another: After the war the national boundaries were redrawn. Boundary is used for the borders between counties in Britain, both on the map and on the ground: We crossed the county boundary into Devon. States and counties in the US are separated by lines but ‘line’ is not used for national borders: crossing state/county lines. Boundary can also be a physical line between two places, for example between property belonging to two different people, marked by a fence or wall: the boundary fence/wall between the properties
PATTERNS AND COLLOCATIONS • across/along/on/over a/the border/boundary/line/ frontier • at the boundary/frontier • the border/boundary/frontier with a place • the northern/southern/eastern/western border/ boundary/line/frontier • a national/common/disputed border/boundary/ frontier • a county boundary/line • to share a border/boundary/frontier • a border/boundary/frontier dispute • a border/frontier crossing/post/area/town