After reading the Hulk's entry at Marvel.com I'm still unsure what exactly Bruce's alternate form is technically classified as. The only description given about the Hulk is "monster", but it's used in different ways:
Multiple Personality Disorder (literal)
Gamma-powered psychiatrist Leonard "Doc" Samson captured the Hulk and successfully separated Banner and his alter ego. Without Banner's psyche to restrain him, the Hulk became a greater danger than ever before. Realizing there was only one way to reign in the Hulk, Banner agreed to merge with the monster.
World War Hulks (figurative)
During the brawl Skaar accidentally placed several innocents at risk. Hulk saved their lives, proving he wasn't the monster Skaar or Banner made him out to be. Father and son finally reconciled and decided to act as a family.
So it seems like the term "monster" isn't really how people in-the-know would consider the Hulk.
So what is he? Is the Hulk a mutation? A limited evolution? An involuntary super power? Is he still considered human? If there are multiple, conflicting possibilities, then the most recent one will do.