The following is pure opinion and nothing more:
In brief, to me, low magic connotates an environment where any magical properties, even small ones have a significant impact on the game play, yet does not infringe upon the effectiveness of the class-specific capabilities built into the default rule set. Since I have only ever played 3.0 in video form, and we never needed to categorize our games in pre-3.0 PnP games, I am totally blind to the impact of other D&D versions, including Pathfinder. Most all my playmates are as equally near-sighted about versions that were never implemented into the default NWN gameplay.
High magic, to me, is any environment that has run amok and has created an artificially unbalanced usage of those same magical adjuncts that worked so well to produce a balanced environment in the less "saturated" case above. These environemnts typically create special monster AIs with elevated saves, SRs, DRs and HP levels near 1K or better, I suppose, to give some relevance to the unmanageable properties of the available items.
Mid magic is a mixed bag where one has no clue what they will experience until they've make a few trips though themselves. The +X categorization has little significance to anew player until he/she experiences the monster and environmental (like uber trap DCs) customizations. It is quite possible for a +10 blanket enhancement to play like low magic if the monster skins have been modified to completely balance the increased enhancements (HPs, AC, DR, etc.). What can tip the mid magic scale toward high magic is the introduction of those properties that imbue class abilities or feats. Once any class asset is compromized or neutralized it has become a high magic environment. So mid magic is a catch-all category which can be much wider than either low or high depending on how the environment has been designed around the enhancement levels.
As far as I am concerned, character level has no direct impact on the relative effectiveness of item enhancements. However, the way many of the prestige classes are designed tends to force a higher magic enhancement level, like SD's HiPS, AA's +15 innate enhancement and PM's crit immunity to name only 3. In epic levels, the capacity of some prestige features and availability of potentially-unbalancing feats (like Epic Dodge, Epic Warding, DevCrit to name 3 of those) may cause a higher magic capacity to be implemented just to "keep pace" with the standard default progression of that potential. But I believe the issue becomes more related to providing equally progressive capabilities across all the classes, inclucing the prestige ones, rather than figuring out what properties to place on items.
God! How can they stand trying to balance PRC worlds? Seems like a nightmare to me.
'>
So now that you've suffered to read this far, we get to my REAL question...
Besides the eternal debate about low/mid/high magic, what other "Jargon" is worth trying to define in this thread?