When I first joined this forum many years ago, I took it that people who straddle the gender divide were called "crossdresser". And then the terms "bigender" or "dualgender" began making an appearance. The term "genderqueer" has been popping in and out of the lexicon too. Over the years, there has been "inbetweener", "pangender", "femulator", "gender-nonconforming" and a slew of others I can't remember at the moment.
Now, I use the term "gender fluid" simply because it is there, it is simple and is all-encompassing, it doesn't label or fall in or out of fashion like the other words. It's a term as good as any to describe someone who does not feel a solid, unchangeable, 24/7 gender identity like "male" or "female", specifically it describes someone who fluctuates between the two.
But, Nadine has brought up the idea that a non-binary gender is not necessarily fluid. It does not necessarily fluctuate. Some people always feel internally the same state of in-between no matter how they dress, and so this is not fluid. I agree.