Twin cities refer to two neighboring cities or urban centers that grow into a single conurbation or narrowly separated urban areas over time. There are no formal criteria, but twin cities are a special case of two cities that have cultural and commercial ties. The term "Twin Cities" is most commonly used to refer to Minneapolis and St. Paul, which are two cities in Minnesota that are located close to each other. However, some people use the phrase as a shorthand for "the Twin Cities metropolitan area," which includes other cities in the region.

A sister city or twin town relationship is a form of legal or social agreement between two geographically and politically distinct localities for the purpose of promoting cultural and commercial ties. Sister Cities International (SCI) is an organization that recognizes sister city relationships when mayors or highest elected officials from the two communities sign off on an agreement to become sister cities. A city may have any number of sister cities, with community involvement ranging from a half dozen to hundreds of volunteers. Sister city organizations can include representatives from nonprofits, municipal governments, the private sector, and other civic organizations. Each sister city organization is independent and pursues the activities and thematic areas that are important to them and their community, including municipal, business, trade, educational, and cultural exchanges and projects with their sister city.