Mesoamerica is the academic designation for all countries occupied by or significantly influenced by the Aztec, Maya, Teotihuacan, Toltec, and Olmec cultures. These were the civilizations that had trade routes al the way from Mexico through Central America. The Olmecs were interested in colors of jadeite they could not get locally in Veracruz or Guerrero, Mexico. The Aztecs were eager for exotic products literally from most of Mexico and far south to Costa Rica. The Teotihuacan merchants and rulers also interacted with Guatemala and Honduras.
We are aware of the importance of the Zapotec, Mixtec, Totonac, West Coast Mexico, and diverse other civilizations. The Guatemalan ruins of Bilbao and Cotzumalhuapa civilization is a great example of how cosmopolitan some cultures were.
Every scholar defines Mesoamerica a bit differently but it includes about 80% of Mexico, all of Guatemala, Belize, El Salvador, Honduras, Nicaragua, and at least part of Costa Rica.