The term "cession" refers to the formal giving up of rights, property, or territory from one entity to another. In the context of Mexican history, specifically the Treaty of Guadalupe Hidalgo in 1848, Mexico ceded a significant portion of its territory to the United States. This included present-day California, Arizona, New Mexico, Nevada, Utah, and parts of Colorado and Wyoming.
In total, Mexico ceded about 55% of its territory as a result of the war and the treaty. If you have a specific context or percentage in mind, please provide more details!