Ethnic nationalism or Ethnicism is a form of nationalism wherein the "nation" is defined in terms of ethnicity. Whatever specific ethnicity is involved, ethnic nationalism always includes some element of descent from previous generations and the implied claim of ethnic essentialism, i.e. the understanding of ethnicity as an essence that remains unchanged over time.

The central theme of ethnic nationalists is that "..nations are defined by a shared heritage, which usually includes a common language, a common faith, and a common ethnic ancestry." It also includes ideas of a culture shared between members of the group, and with their ancestors, and usually a shared language; however it is different from purely cultural definitions of "the nation" (which allow people to become members of a nation by cultural assimilation) and a purely linguistic definitions (which see "the nation" as all speakers of a specific language). Herodotus is the first who stated the main characteristics of ethnicity, with his famous account of what defines Greek identity, where he lists kinship (Greek: ὅμαιμον - homaimon, "of the same blood"), language (Greek: ὁμόγλωσσον - homoglōsson, "speaking the same language"), cults and customs (Greek: ὁμότροπον - homotropon, "of the same habits or life").