What type of weapon is Nnoitra's zanpakuto?
Nnoitra's zanpakuto, Santa Maria, is unique in regards to what type of weapon it is. 

Unreleased, it resembles an axe with two crescents or a halberd, with it's edges rotated by 90 degrees.



But due to the placement of the blade edges, it resembles a scythe more so than an axe.

When released, it resembles a scythe:

 

But note that when he was the 8th-ranked Espada, his weapon resembled a staff with a giant crescent:



Note that it does a similar motif to the Chinese monk's spade (aka Crescent Moon Spade) weapon that's like a halberd.