A border is a line that marks where one place ends and another begins. Countries have borders that separate them from their neighbors: the border between the United States and Canada stretches thousands of miles, while the border between Spain and Portugal divides the Iberian Peninsula. States, counties, and even your own backyard can have borders that define their boundaries.

Borders can be marked in different ways. Some follow natural features like rivers or mountain ranges. The Rio Grande forms part of the border between the United States and Mexico. Other borders are simply invisible lines agreed upon by governments, marked on maps but not visible on the ground except for signs or fences. When you cross a border between countries, you usually need to show your passport and sometimes go through customs.

The word can also mean an edge or boundary of something smaller. A garden might have a border of flowers around it, or a page might have a decorative border framing the text. When something borders on being rude, it comes very close to crossing the line into rudeness without quite getting there.

As a verb, border means to share a border with: “Tennessee borders eight other states, more than any other state except Missouri.”