The stage of the Persian language that follows Middle Persian, formally beginning with the Islamic conquest (8th century), and first attested at the end of the 9th century. Distinguished from Middle Persian by extensive borrowing of vocabulary.
1975: Gilbert Lazard, "The Rise of the New Persian Language," Cambridge History of Iran, vol. IV, p. 597.
It is particularly in its vocabulary that New Persian departs from Middle Persian, in two different ways. First, New Persian contains many words which are originally foreign to Persian proper and are betrayed by their form as belonging to other, northern or eastern Iranian dialects: New Persian owes much to Parthian and to related dialects and it has also borrowed from Soghdian. Secondly, it has admitted a considerable proportion of Arabic words, a proportion which has increased with time [...]
1986, Lawrence Paul Elwell-Sutton, "Arabic Language III", Encyclopedia Iranica, vol. 2, p. 234.
the earliest samples of New Persian consist of a handful of verses dating from the end of the 3rd/9th century, by which time the influx [from Arabic] had been going on for two centuries.