Tajiks are a Persian-speaking Iranian ethnic group native to Central Asia, living primarily in Afghanistan, Tajikistan, and Uzbekistan. Tajiks are the largest ethnicity in Tajikistan, and the second-largest in Afghanistan and Uzbekistan. They speak varieties of Persian, a Western Iranian language. In Tajikistan, since the 1939 Soviet census, its small Pamiri and Yaghnobi ethnic groups are included as Tajiks. In China, the term is used to refer to its Pamiri ethnic groups, the Tajiks of Xinjiang, who speak the Eastern Iranian Pamiri languages. In Afghanistan, the Pamiris are counted as a separate ethnic group.

Persian, also known by its endonym Farsi or Parsi, is a Western Iranian language belonging to the Iranian branch of the Indo-Iranian subdivision of the Indo-European languages. Persian is a pluricentric language predominantly spoken and used officially within Iran, Afghanistan, and Tajikistan in three mutually intelligible standard varieties, respectively Iranian Persian, Dari Persian, and Tajiki Persian. It is also spoken natively in the Tajik variety by a significant population within Uzbekistan, as well as within other regions with a Persianate history in the cultural sphere of Greater Iran. It is written officially within Iran and Afghanistan in the Persian alphabet, a derivative of the Arabic script, and within Tajikistan in the Tajik alphabet, a derivative of the Cyrillic script.