The word comes from the verb ista- ("to know"), and possibly the agentive ending -ro.
Quenya
Istar
wizard
istari
Istari
curuvar
wizard
curuvar _("k")_noun "wizard" (LT1:269 but Gandalf, Saruman etc. were istari)
sairon
wizard
sairon noun "wizard" (SAY); according to LT2:337 and GL:29, Sairon is also the Quenya (or Qenya) name of Dairon (Daeron).
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.