Central America is generally thought of as the narrow strip of nations that connect North and South America. Mexico's land mass is pretty wide, especially to the north, negating it from the above description. The southern portions of Mexico are sometimes considered part of this narrow corridor, but it wouldn't make sense to split up Mexico, with part being in North America and part being considered Central America.

1) It is a question of how you define it. Various interest could play a role in the definition. Actually, for various countries or organizations, Mexico (in whole or part) is considered a part of Central America. 2) ""Central America" may mean different things to different people in the world according to the context: - In English, Central America is considered a region of the North American continent. Geopolitically, it usually comprises seven countries – Belize, Guatemala, El Salvador, Honduras, Nicaragua, Costa Rica, and Panama. Mexico, in whole or in part, is occasionally included. Some geographers include the five states of Campeche, Chiapas, Tabasco, Quintana Roo, and Yucatán, together representing 12.1% of the country's total area. - In Latin America, Iberia, and some other parts of Europe, the Americas are considered to be a single continent, and Central America is considered a region of this continent. In Ibero-America, the region is defined as seven nations – Belize, Guatemala, El Salvador, Honduras, Nicaragua, Costa Rica, and Panama – and may occasionally include Mexico's southernmost region. Geopolitically, Mexico is not considered part of the region. - The UN geoscheme defines the region as all states of North America south of the United States; conversely, the European Union excludes Belize and Mexico from its definition of the region. - In Central America, the concept is divided in "Centroamérica" and "América Central". "Centro América" is used as a historical term to define the countries that formed part of the Federal Republic of Central America, while "América Central" is a contemporary concept of Central America as a region, now including Panama and Belice. This means, "Centroamérica" is a political term and "América Central" a geographical term. " Source and further information: http://en.wikipedia.org/wiki/Central_America#Usage 3) "Middle America is a region in the mid-latitudes of the Americas. In southern North America, it comprises Mexico, the nations of Central America, and the West Indies. Using this geographical model, the region of Northern America is to the north, while South America is to the southeast. Sometimes, Colombia and Venezuela are also included in Middle America." Source and further information: http://en.wikipedia.org/wiki/Middle_America_(Americas)

Mexico and Central America are all a part of North America. The way I've always thought of it is not by a collection of countries considered to be Central American countries, but rather just a geographical area, the one that connects the greater part of North America to South America. In the picture below, I've circled what I consider (roughly) Central America to be in red. It includes all of the countries generally considered to be Central American, and also part of southern Mexico.

Geography is sometimes not logical; it too has lingering names due to long time cultures that have established themselves in those areas. Although, Central America is considered to be part of the North American Continent; the best reasoning to why not to consider Mexico as part of Central America is by looking at the tectonic plates for that area: en.wikipedia.org/wiki/File:Tectonic_plates_Caribbean.png Mexico lies on the North American Plate; most of Central America lies on the Carribbean Plate. Further, it is reasonable to accept that Central America should not be labeled as part of the South American Continent as it also floats above the equator.