Persians represents the largest ethnic group in Iran, 33 million according to 2003 estimates, which is about half of the country's entire population. Persians dominate in dispersed areas across Iran, except in the west and the southeast.
Depending on definitions, there are several millions living in other countries, both in neighbouring countries to Iran, as well as large immigrant communities in America, Europe and Australia.
The reason why figures of Persians in other countries are uncertain, is because other peoples are sometimes grouped together with them, according to classifications defined by Western researchers with limited direct knowledge from Iran. The distinctions between them are defined in several ways, according to ethnicity, language and history.
Several peoples are defined as Persian subgroups. The largest among these are the Tajiks, who form the great majority of Tajikistan. Another group, the Farsiwan of Afghanistan may be understood as being Tajiks, or an independent Persian subgroup. Hazara and Aimak, also of Afghanistan, are groups that can be identified as Persians, although their ethnic origins are quite separate from the main Persian groups.
The oldest known traces of the name "Persian" was from the 1st millennium BCE, when the term Parsua was in common use across the larger Middle East. In Greek language this would became Persis, from which the Western term Persian has been derived. The name the Persians of the earliest periods used for themselves was Aryan. The modern term in both Persian and Arabic, Farsi, is often explained from the mere lack of the letter 'p' in the alphabet used with the Persian language; the Arabic alphabet. However, this is at best an insufficient explanation, as 'b' rather than 'f' is the common replacement for the p-sound.
One very common definition of Persians is by their speaking the Persian language.
A clear majority of Persians are Shi'i Muslims, a large minority are Sunni. Other religious groups include Alevism, Isma'ilism, Zoroastrianism, Christianity, and Baha'i.