Well, ideally I'd say that 1-2 are games that are horrible, 3-4 are games that are mainly bad/boring but you can at least get some enjoyment out of, 5 is average/pretty good and 6-10 are good games.

6.5

7 is average, because that's what most game reviewers use as the average.

Depends on reviewer, outlet, and scale. Generally in gaming they use the 1-5 = bad, 6 poor, 7 okay, 8 good, 9 great, 10 excellent system.

7.

A game with this rating tells me that the game works and functions completely fine, but nothing about the game will impress you or anything either.

It's impossible to extrapolate to encompass all reviewers, the way I did was if I was scoring the games myself.

I think the problem here is the phrasing. 5 is not average. 5 is not average by any definition. Looking at all scores this gen the average is actually above 7. So 5 really is quite below average. Game quality has been steadily increasing. Since mass ratings started. I can't really blame anyone calling a 7/10 average. 7 is good and most games are quite competent so giving good games that don't stand out much a 7 is absolutely fine.

Most games that even get reviews are already a preselection of the better games.

So 5/10 is an average game when you include all the shovelware crap, but 6/10 or 7/10 is the average of whats left after filtering out the really bad games.

6.9 or above

Metacritic's average seems to hover around 7 so I'll go with that.

Old data but it might still be valid
http://www.polygon.com/2014/10/28/7083373/look-at-this-chart-of-average-metacritic-scores-what-happened-in-2007