The results of people of targeted racial groups (Blacks, Lations/Hispanics, Asians, Native) believing, acting on or enforcing the dominant (White) system of racist discrimination and oppression. Horizontal racism can occur between members of the same racial group or between members of different, targeted racial groups.
Horizontal racism occuring between members of the same racial group: An Asian person telling another Asian who is wearing a Sari to dress like an "American". A Latino person telling another Latino to stop speaking Spanish.
Horizontal racims occuring between members of different targeted racial groups: Latinos believing stereotypes about Native Americans, Blacks not wanting Asians to move into predominantly Black neighborhoods, etc.