a line that indicates a boundary
"The new fence was built along the border of the two properties to clearly mark where one yard ended and the other began."
The outer edge of something.
"The hikers carefully followed the river's border to stay within their designated campsite."
In plain English: A border is the line that marks where one place ends and another begins.
"The old fence clearly marks the border between our property and the neighbor's yard."
Usage: Use "border" to refer to the line or strip that marks the edge of an area or object, such as a garden border or a national border. Do not use it to mean the act of touching or being adjacent to something, which is the function of the verb phrase "to border on."
To put a border on something.
"She decided to buy gold leaf to border the edges of her handmade greeting cards."
In plain English: To border something means to touch or be right next to it without going over the edge.
"The new garden borders the old forest."
Usage: Use "border" as a verb to describe two things touching or meeting along an edge, such as forests bordering a city. Do not use it to mean adding a decorative frame around an image, which requires the phrasal verb "border on" only in the sense of approaching a limit or topic.
A surname.
"My neighbor, Mr. Border, invited us over for dinner last night."
The word "border" comes from Middle English and Old French roots that originally referred to an edge or trim. It traveled into modern usage as a synonym for the boundary line between two areas.