Ethnogenesis is the process by which a group of people comes to be viewed, or comes to view themselves, as a new ethnic group, distinct from other ethnic groups in their environment.
Linguistic differentiation is often an important part of ethnogenesis, because if people come to view their mode of speaking as a different language (rather than merely a separate dialect) from that used by those around them, ethnogenesis is very likely to follow, and conversely, if leaders seek to foment ethnogenesis, it is often very important to reclassify a dialect as a separate language. This linguistic differentiation also often involves the "discovery" or outright creation of a special "ethnic" literature.
Ethnogenesis also often involves the creation or enhancement of a shared ethnic history, shared cultural traditions, shared ethnic clothing, shared ethnic cuisine, etc.
Ethnogenesis can be either passive or active. "Passive" or "natural" ethnogenesis involves a gradual shift over time toward a view that a certain group is a separate ethnicity with a distinct ethnic experience, due to factors such as systematic persecution, extreme social stratification, geographic isolation, real linguistic change, etc.
On the other hand, "Active" ethnogenesis occurs when leaders, governments, or intellectuals deliberately "engineer" a new ethnicity into existence, often to solve an existing political problem, or make ethnic boundaries conform more closely to preexisting political boundaries. This kind of ethnogenesis often takes place hand in hand with the creation of new nation states, although generally not in the case of the creation of new "state nations", such as the United States.