Originally denoting a person of mixed white and indigenous race in former Spanish colonies in Latin
America and the Philippines. This is not strictly the case anymore.
Nowadays, particularly in Latin
America, it really denotes any mainstream Latino/a, regardless of their actual ancestry. Oftentimes, indeed, perhaps most of the time, people who are purely American Indian by race are called "mestizos" simply because they speak Spanish, do not have any tribal affiliation, and are otherwise just seen as a mainstream Mexican/Peruvian/Ecuadorian, etc. The only people termed "Indians" nowadays in Latin
America are those who maintain an ethnic identity apart from the mainstream, speak a non-Spanish language, etc.
In the Philippines, unlike Latin
America, "mestizo" has maintained its mostly racial definition.