Some people have mentioned spears and pole arms, but those are a general class of weapon. There are many variations on pole arms and spears. Spears typically have a double edge, and pole arms often have the spear head flanked by double or single axe heads, so I did a bit of research on Wikipedia. What makes Whitebeard's weapon stand out is the lack of an axe head and the single sided blade, effectively making it a sword blade on a pole. So my conclusion is that Whitebeard is wielding a Japanese "naginata". The European equivalent would be a glaive.
What is interesting is that Oda chose to give Whitebeard a weapon that, according to Wikipedia, has become associated with women and in modern Japan is studied more by women than men.
Wrong, as you can see Naginatas blade is like a katana, small, and Whitebeards blade isnt small, so it is probably a guan dao/ Kwan dao, the famous weapon that the warlord Guan Yu used.
This does acttually fitt very weel, as they said Guan Yu was the most famous warriors at his time in China, kinda like Whitebeard now.
I hope though that we will see a strong female with a naginata… That would rock...
And you have wrong on another point as well, spear is a polearm, just like sabre and katanas are swords. Polearms are spears, glaives, pikes, halbeards, lances, naginatas, Yaris, qiangs, Guan daos and so on.. understand?
I'm a weapon nerd... ^^