The “Mexican cession” was the territory Mexico called the “Far North,” including what are today California, Nevada, Utah, most of New Mexico and Arizona, and parts of Wyoming and Colorado.
In return Mexico received $15 million, was set free of $3 million in American claims, and got rid of American forces occupying its capital.
The land cession was part of the Treaty of Guadalupe Hidalgo on February 2, 1848, ending the Mexican War.