Tex-Mex is a term used to describe Americanized Mexican food, particularly from the south or southwest of the United States.
"Tex-Mex" first entered the language as a nickname for the Texas-Mexican Railway, which was chartered in 1875.  In the train schedules published in newspapers of the 1800s, the names of railroads were abbreviated. The Missouri Pacific was called the Mo. Pac., and the Texas-Mexican was abbreviated Tex. Mex.
In the 1920s, the hyphenated form was used in American newspapers in reference to the railroad and to describe people of Mexican descent who were born in Texas.
In the mission era, Spanish and native American foods were combined in Texas as in other parts of New Spain.
The Spanish influence on cuisine in Texas began with the arrival of Juan de Onate with six hundred colonists and seven thousand head of livestock in El Paso on April 20 1598. The cuisine that would come to be called Tex-Mex actually originated with the Tejano people as a hybrid of Spanish and Native American cooking. The Tejanos didn't immigrate to Texas from Mexico, rather they were native Americans who were educated in Spanish missions in San Antonio and elsewhere in Texas.
Native American contributions to Tex-Mex include pecans, pinto beans, nopales, wild onions, mesquite meal, and agarita berries.
Canary Islanders made important contributions to early Tex-Mex as well. Families of colonists who immigrated to San Antonio from the Canary Islands at the invitation of the Spanish crown in 1731 are credited with bringing a Berber influence to Texas food. Their taste for profuse amounts of cumin, chillis and coriander (cilantro) contributed to the Tex-Mex flavor signature, which differs significantly from that of interior Mexican food. A New World version of the highly spiced Morrocan meat stew called tangia made by Canary Island women in outdoor cooking pots in San Antonio may have been an early forerunner of chili con carne, a classic Tex-Mex dish.
Tex-Mex was subsequently influenced by the cooking in neighboring states of Mexico. The ranching culture of South Texas and Northern Mexico straddles both sides of the border. A taste for cabrito (kid goat), barbacoa (barbecued cow heads), carne seca (dried beef), and other products of cattle culture are common on both sides of the Rio Grande.
In the twentieth century, Tex-Mex took on such Americanized elements as yellow cheese as goods from the United States became cheap and readily available. Diana Kennedy, an influential food authority, first delineated the differences between Mexican cuisine and Americanized Mexican food in her 1972 book The Cuisines of Mexico.
The first use in print of "Tex-Mex" in reference to food occurred in the Mexico City News in 1973.
Award-winning Texas food writer Robb Walsh updated Kennedy and put her comments regarding Tex-Mex cooking into historical and sociopolitical perspective in The Tex-Mex Cookbook: A History in Recipes and Photos (New York: Broadway Books, 2004).
The ingredients used are common in Mexican cuisine, although ingredients unknown in Mexico are often added. Tex-Mex cuisine is characterized by its heavy use of meat (particularly beef), beans, and spices. Nachos, crispy tacos, crispy chalupas, chili con queso, chili con carne, chili gravy, and fajitas are all Tex-Mex inventions. Serving tortilla chips and a hot sauce or salsa as an appetizer is also an original Tex-Mex combination. Moreover, Tex-Mex has imported flavors from other spicy cuisines, such as the use of cumin (common in Indian food), but used in only a few authentic Mexican recipes.