>Presuming you mean Farsi
I've been meaning to look into this after it came up in a dinner conversation a while back. You reminded me and I just found this from Iran's Cultural Heritage News Agency http://www.chn.ir/en/About/
19 November 2005 -- Persian (Iranian) people in the English-speaking countries are the only community who use two different terms to refer to their language, "Farsi" and "Persian." This behavior has caused some confusion among the Westerners as to the appropriateness of these terms.
[...]
"Farsi" (an Arabic adaptation of the word "Parsi"), is the indigenous name of the Persian language. Just as the German speaking people refer to their language as 'Deutsch', the Greek 'Ellinika' and the Spanish 'Espanol', the Persians use 'Farsi' or 'Parsi' to identify their native form of verbal communication.
In English, however, this language has always been known as "Persian" ('Persane' in French and 'Persisch' in German'). But many Persians migrating to the West (particularly to the USA) after the 1979 revolution continued to use 'Farsi' to identify their language in English and the word became commonplace in English-speaking countries.
[....]
Some mistakably believe that, in English, the official language of Iran should be called "Farsi," while the language spoken in Tajikistan or Afghanistan should be labeled as "Dari," and "Persian" should be utilized to refer to all of them! However, the difference between the Persian spoken in Iran, Afghanistan, or Tajikistan is not significant and substantial enough to warrant such a distinction and classification. Consider the following case. An Egyptian and a Qatari engage in a conversation in Arabic. They will encounter a great deal of difficulties in comprehending each other. Despite this fact, however, the language used in their conversation is referred to as "Arabic." No one will even attempt to classify their respective dialects separately and refer to them as "Qatari" and "Egyptian"! On the other hand, Persians, Tajiks or Afghans can converse in Persian and easily understand each other. Then, why should their dialects be classified separately and referred to by different names?
In English, usage of "Farsi" in place of "Persian," that has been common since 1980s, is as inaccurate and odd as using "Farsi Gulf" instead of "Persian Gulf."