Genízaro was a term used in eighteenth- and nineteenth-century New Mexico for "detribalized Indians," a variety of individuals of mixed Native American, but not Pueblo, parentage who had adopted at least some Hispanic styles of living. They were most common in areas of New Mexico adjacent to the Southern Plains. Genízaros, many of whom were descendants of Native Americans who made their home in the Great Plains, are a little-studied group. They appear to have been a transitional group that appeared and then disappeared as part of the opening, and later closing, of a particular set of frontier relations in New Mexico. Even the origin of the term genízaro is controversial.

The more commonly claimed origin is from the term for captive Christians who were forcibly converted to Islam and served as troops in the Turkish army, called yeni-cheri, anglicized as janissary. Because of the close phonetic equivalence and because of similar roles played by genizaros in New Mexico, this is assumed to be the genesis of the term. Steven Horvath argues persuasively for a different origin. To the root geno-, meaning lineage or race, are added suffixes -izo and -aro, yielding the Spanish word. It referred to people who were the children of parents from two nations, for example France and Spain, or in New Mexico, Comanches and Pawnees. Writing in 1872, Fray Juan Augustin Morfi explained, "This name is given to the children of the captives of different nations who have married in the province." The term became generic for Native Americans who had been born among nomadic groups, but who lived in New Mexico.

There were two sources of genizaros. First, they might have been taken captive in the many fights with surrounding nomadic groups. Second, they might have been traded (or "rescued") from friendly Indian groups who had taken them captive in their raids on enemies. Typically, they were children or women and were used as servants and laborers. To skirt the legal ban on slavery, they were officially designated as under the protection of a Spanish household, whose head was to train them in Christianity and Spanish culture generally. In practice they were often slaves. They were desirable because of labor shortages, especially in frontier areas. Consequently, their initial commonality was that they had been born Native Americans but lived in Hispanic society and occupied the lowest social strata. Because they often grew up in captivity, they knew little of their natal culture and hence were often described as "detribalized Indians."

Some genízaros eventually earned their freedom and worked as day laborers; a few became landowners or craftsmen. Two avenues to improved status were open to them. They could settle new areas where the Spanish government sought to expand control. Also, men could serve in militia units to fight hostile nomadic Indians. Because of their origins their loyalty was suspect, but they were deemed particularly adept at dealing with or fighting nomadic Indians because of their putative fierceness and because some had at least rudimentary knowledge of one or more languages of nomadic Indians. (This same quality allowed them to participate in locally lucrative but illegal trade with Plains Indians.) Their military role is one basis for the common assumption that genízaro was a Spanish term for janissary.

Genízaros who stood out in battles eventually could own land or enter occupations other than day laborer or soldier. Land grants to genízaro settlers typically were along frontiers where fighting was heaviest. Through time others of ambiguous ancestry might join such communities. Thus, genízaro came to refer to anyone of ambiguous ancestry and/or lower status. Eventually, more successful genízaros passed into the general Hispanic population. By the late nineteenth century the term gradually fell into disuse. The term is seldom used today except by historians and genealogists studying New Mexico history.

Determining the number or proportion of genizaros is difficult. First, it was a status individuals sought to hide. Second, it changed through time for individuals and for families. Finally, as both social relations and the terms for various groups evolved, just who should be counted as genizaro changed. Estimates range from less than 10 percent of the settled population to as high as one-third–enough to be an important social component of Hispanic society in colonial New Mexico, especially in its interactions with Native Americans who lived in the Great Plains.

For some individuals genizaro might be a multigenerational transitional status in a passage from Indian to Spaniard. Collectively, they were a buffer group created by the combination of a need for labor and a supply of captives from nomadic Indians. When the flow of captives slowed, and finally ceased, the group was no longer refreshed by new members and gradually disappeared.