The iroha (いろは) ordering is a kana ordering based around a Buddhist poem which contains each of the Japanese syllables exactly once. This ordering is still used today, and it is not unusual to see items numbered i, ro, ha where English would use A, B, C, etc. However, in dictionaries and other lists of words, it is much less common nowadays than the more regular gojuuon ordering. (See 4.1.4. What is the origin of the gojuuon kana ordering? )
There are actually several different versions of the ordering corresponding to different versions of the poem. The most common one is
i ro ha ni ho he to
chi ri nu ru (w)o
wa ka yo ta re so
tsu ne na ra mu
u (w)i no o ku ya ma
ke fu ko e te
a sa ki yu me mi shi
(w)e hi mo se su