As for the accusation that I'm being arcane - all I can do is point you to that most un-arcane and accessible of all reference works, Wikipedia. It spells it out quite clearly, and agrees with me entirely.
2. Pointing out that something is on Wikipedia does not prove how un-arcane it is. I'm sure there are plenty of topics on Wikipedia that are only understood by a small group of people. Wikipedia doesn't require that all pages be common knowledge to the general population before being accepted.
The simple fact is that "average" is a general term which is only used correctly once you've clarified what your exact measure is.
This is especially easy to see when people talk about "average income", since it's clearly skewed by there being a long tail on the high-income side which pushes up the average salary. There's a reason why income is generally bracketed instead, and split into quintiles.
Dig up arcane usages to support your confusing wording all you want, but the point of language is communication, and when you use words in significantly different ways from other people, you're communicating unclearly.