Mestizo is a term traditionally used in Spain and Spanish America to mean a person of combined European and Amerindian descent, or someone who would have been deemed a Castizo (one European parent and one Mestizo parent) regardless if the person was born in Latin America or elsewhere. The term was used as an ethnic/racial category in the casta system that was in use during the Spanish Empire's control of their New World colonies. Mestizos are usually considered to be mixed Spaniards by the crown of Spain.
The term mestizaje, taking as its root mestizo or "mixed", is the Spanish word for the general process of mixing ancestries. In English the term is miscegenation.
To avoid confusion with the original usage of the term mestizo, mixed people started to be referred to collectively as castas.
During the colonial period, mestizos quickly became the majority group
in much of the Spanish-speaking parts of Latin America, and when the
colonies started achieving independence from Spain, the mestizo group
often became dominant. In some Latin American countries, such as Mexico,
the concept of the "mestizo" became central to the formation of a new
independent identity that was neither wholly Spanish nor wholly
indigenous, and the word mestizo acquired its current meaning of dual
cultural heritage and descent.