What is the threshold between high Aspect Ratios and low Aspect Ratios? Oftentimes when describing flying organisms and vehicles, their wings are said to have a "high aspect ratio" or a "low aspect ratio". Sometimes the papers (that I read) gave the numbers, but more often, they would just leave the explanation at that. 

So I was wondering, in general, what is the minimum ratio that would make it a "high AR" and what is the minimum ratio that would make it a "low AR"? Is there a threshold to be met, or is this analysis of AR rather relative and or subjective?
The boundary depends on the type of aircraft. Fighter wings will be called high aspect ratio when the same aspect ratio in a glider wing will still be called low.

Generally, values below 5 are low, regardless of aircraft type.

With passenger aircraft, the boundary might be around 8.

Gliders will only have a high aspect ratio if it surpasses 20.

Still, there is no defined boundary and in the end the choice depends on the whims of the author.