Rav akçesi was a "rabbi tax" paid by Jewish communities in the Ottoman Empire.
[1][2] The origins of rav akçesi are unclear; it has been suggested that it was one of two taxes imposed specifically on Jews, and that it may have developed in parallel with the authority of a senior rabbi in Istanbul, who was at nominally a representative and judge for Jewish communities in the Ottoman empire, although their authority may not have extended far beyond Istanbul.
[3][4] It has been suggested that Mehmet II imposed the tax in return for separate representation of Jews after 1455, as part of a broader effort to rebuild and revive Istanbul; this may also have served to undermine the Greek patriarchy.
[7] The Porte was well aware of this - and even aware that this would tempt non-Muslims to convert; Bayezit II ruled that courts should treat non-Muslims more leniently (including such measures as lower fines), "so that the poll-tax payers shall not vanish".
[8] As with other taxes in the Ottoman empire, rav akçesi could be affected by a complex patchwork of local rules and exemptions, including muafiyet; the Jews of Selanik (Thessaloniki) were among those exempted from taxes by a muafname after the city was conquered by Murad II.