Final answer:
A boundary is an invisible line that separates political entities on Earth. There are two main types, formal boundaries like the border between countries, and functional boundaries such as the service area of a public transportation system.
Step-by-step explanation:
A boundary is an invisible line on the Earth's surface that divides the surface area into distinct political entities. There are two primary types of boundaries: formal boundaries and functional boundaries. A formal boundary refers to lines that have governmental, administrative, or political significance, and they are typically not open to dispute.
For example, the border between the United States and Canada is a formal boundary that separates the two countries. On the other hand, a functional boundary is often based on how a certain area is used or functions within a larger context. For instance, the service area of a public transportation system can be considered a functional boundary as it describes the extent to which the transportation services are offered.