The term Trans-Pecos, as originally defined in 1887 by the Texas geologist Robert T. Hill, refers to the portion of Texas that lies west of the Pecos River. The term is considered synonymous with "Far West Texas", a subdivision of West Texas. The Trans-Pecos lies within the Chihuahuan Desert, the most mountainous and arid portion of the state, and it is well known for its scenic desert vistas, such as Big Bend National Park and the Guadalupe Mountains.

The Trans-Pecos region is divided into nine large counties: Brewster, Culberson, El Paso, Hudspeth, Jeff Davis, Pecos, Presidio, Reeves, and Terrell counties (See table below). Brewster County, the largest, has a land area of 6,193 sq mi (16,040 km²). To put this figure in perspective, consider that the land area of Brewster County is approximately 10% larger than the entire state of Connecticut, yet it has a population density of only 1.4 persons per square mile compared to the 723 persons per square mile that reside in Connecticut. However, Brewster County is not the least populated of the nine counties that comprise the Trans-Pecos. Terrell County, which borders both the Pecos River and the Rio Grande, is the least populated county in the Trans-Pecos region. According to the United States Census of 2000, Terrell County had a total population of 1,081 residents occupying a land area of 2,358 sq mi (6,107 km²) resulting in a population density of less than 0.5 persons per square mile.