Tajik or Tajiki (Tajik: забо́ни тоҷикӣ́, zaboni tojikī, [zaˈbɔni tɔdʒiˈki]), also called Tajiki Persian (Tajik: форси́и тоҷикӣ́, forsii tojikī, [fɔrˈsiji tɔdʒiˈki]), is the variety of Persian spoken in Tajikistan and Uzbekistan by Tajiks. It is closely related to neighboring Dari Persian with which it forms a continuum of mutually intelligible varieties. Since the beginning of the twentieth century and independence of Tajikistan from Soviet Union, Tajik has been considered by a number of writers and researchers to be a variety of Persian. The popularity of this conception of Tajik as a variety of Persian was such that, during the period in which Tajik intellectuals were trying to establish Tajik as a language separate from Persian language, Sadriddin Ayni, who was a prominent intellectual and educator, made a statement that Tajik was not a "bastardized dialect" of Persian. The issue of whether Tajik and Persian are to be considered two dialects of a single language or two discrete languages has political sides to it (see Perry 1996).
By way of Early New Persian, Tajiki Persian, like Iranian Persian and Dari Persian, is a continuation of Middle Persian, the official religious and literary language of the Sasanian Empire (224–651 CE), itself a continuation of Old Persian, the language of the Achaemenids (550–330 BC).
Tajik is the official language of Tajikistan. In Afghanistan (where the Tajik people minority forms the principal part of the wider Persophone population), this language is less influenced by Turkic languages, is regarded as a form of Dari and as such has co-official language status. The Tajik of Tajikistan has diverged from Persian as spoken in Afghanistan and Iran due to political borders, geographical isolation, the standardization process and the influence of Russian and neighboring Turkic languages. The standard language is based on the northwestern dialects of Tajik (region of old major city of Samarqand), which have been somewhat influenced by the neighboring Uzbek language as a result of geographical proximity. Tajik also retains numerous archaic elements in its vocabulary, pronunciation and grammar that have been lost elsewhere in the Persophone world, in part due to its relative isolation in the mountains of Central Asia.
(Courtesy of Tajik Language Wikipedia)