Here's why:

* Latitude: The Caribbean islands are located between the Tropic of Cancer (approximately 23.5 degrees north latitude) and the equator. This puts them squarely within the tropical zone.

* Climate: The Caribbean experiences a tropical climate characterized by warm temperatures year-round, high humidity, and significant rainfall.

* Biodiversity: The region's diverse ecosystems, including lush rainforests, coral reefs, and mangrove swamps, are all characteristic of tropical environments.

While some may refer to it as a specific geographical zone (e.g., the Caribbean Plate), it is primarily a cultural and historical region. The term "Caribbean" encompasses the islands, the mainland coastal areas, and the waters that surround them.