Oculolinctus is the act of licking a partner's eyeball for sexual arousal. While the practice is somewhat rare, it is a predominantly female one; that is, more women want to do it than men. In the rarest of cases, women have been documented that need to lick the eyeball of their lover in order to acheive orgasm.
The origin of this kink is not clear, though it seems the attraction of licking a lover's eyeball is the trust and intimacy implied in the act.
Should you like to try it, remember that direct contact to the iris may be significantly more awkward or painful than licking the side of the eyeball. One should remember to liberally rinse their mouth with water, so as not to transfer spice or citrus or other painful foodstuffs to the eyeball. Also of note: oral herpes can be transferred through the eye.