A name for the five Wizards who travelled from the West to aid the free peoples against Sauron (LotR/1084, UT/388). It usually appeared in its plural form Istari, which meant “those who know” (Let/202, PM/360). Tolkien generally translated the name as “Wizard”, because the Old English sense of word was similar to the true meaning of this Quenya word (Let/207). The name Istar was most likely a combination of the verb ista- “to know” and the old agental suffix -r(o).
The title Istar was not used for ordinary mortal “wizards” or “magicians” (UT/388). For this purpose, perhaps the word [ᴹQ.] sairon was used instead.
Istar noun "Wizard", used of Gandalf, Saruman, Radagast etc. Pl. Istari is attested. Gen. pl. in the phrase Heren Istarion "Order of Wizards" (UT:388). "The istari are translated wizards because of the connexion of wizard with wise and so with witting and knowing" (Letters:207); by this translation Tolkien tries to reproduce the relationship between Quenya istar and ista- #1, 2.