Server population = amount of people online at a given point in time.
Average server population = amount of people over a given period of time.
The amount of players online varies between 0 and ~2000 per continent (or to be exact, per 2000 per sanctuary (warpgate).
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So that means currently a server can have up to ~6000 players (3x 2000). Now it depends on what is defined as low, medium or high.
I would say low to be 0-40% of capacity used, medium 40-75% of capacity used and high 75-100% of the total potential server capacity. But nobody knows which standards SOE has for Low-Medium-High.