A demigod is a divine or supernatural being in classical mythology. The term has been used in various ways at different times and can refer to a figure who has attained divine status after death, a minor deity, or a mortal who is the offspring of a god and a human.

Now since the Avatar is the result of a human and a spirit merging, thus making the Avatar half-spirit, does this mean that the Avatar fits within the demigod class?

yes

2551289 But would you say it fits any predisposed definition of demigod?

2551334 Supernatural(talks to spirits for the love of god) divine(can use all for elements and can give people there bending back or take them away) but technically they were born with it so I would guess...the more lines of god meet mortal now go to bed.
But meh, it's up to the person.