Bob Myers: The content here has been incorporated into the main page where appropriate, but I am leaving the discussion here to serve as a historical record or in case I bollixed someone's comment.
I've gathered some examples from the web. The conclusion is that one major meaning is indeed turning point, but the (related?) meaning of "intersection of two moyos" is also found. I've added this to the main page.
Of course, the paucity of go material on the web makes it a poor representative sample. However, judging just from these two examples, which are both professional commentary on pro games, perhaps tennozan occurs between move 20 and 80. It's a strategic tipping point right on the boundary of the fuseki and the middle game. Makes sense, when you recall that this is the part of the game where pros spend two hours on a move.
Then again, we have the following, although they are all from pages put up by amateurs:
However, overall usages of tennozan referring to a move are far fewer than the "crucial game of a series" meaning.
unkx80: Both tennozan and shoubute share the same characters as their Chinese counterpart. My understanding from the way Chinese literature presents these terms is that tennozan is the most strategic point on the board at that particular situation, it is some point both plays will want to occupy to get a lead. In this sense, it includes both the intersection of two competing moyos and some vital attack and defense points, often a capping play versus a capping play. Also, missing the tennozan does not imply losing the game in the amateur sense. On the other hand, shoubute often indicates a do-or-die move, by the player who is behind because he judges that he will lose the game if he plays normally to the end, hence the need for an extraordinary move referred to the shoubute.
Bill: My understanding is as unkx80 says. In English, strategic point is probably not strong enough, but at the same time, turning point seems off the mark as far as go usage is concerned. Bob's examples are great, but go more to show that 天王山 is not restricted to the frontier between moyos than anything else.
John F. Shoubute is easily dealt with: shoubu is being used here in the sense of gambling (as in shoubushi). It means a do-or-die/all-or-nothing/meltdown move or similar depending on your idiolect.
But Tennouzan is not a technical term. It is part of everyday language. The difficulty comes from the fact that is has two rather different meanings, both used in go. The commonest "real life" meaning nowadays seems to be as an abbreviation for Tennouzan no tatakai, meaning a decisive battle. It is common in league type sports, e.g. soccer or baseball, to denote, say, a match between the top two teams to decide first place.
The other meaning is commoner in go. It means a commanding position, which naturally comes from the Battle of Tennouzan when the side that occupied this hill (Mount Tennouzan is hardly accurate - it's only about 300 metres high) was able to win because of this strategic advantage. There is even an oblique go connection to this usage because the victor was Hideyoshi who defeated Akechi Mitsuhide, having chased after him when he (Mitsuhide) killed Oda Nobunaga on the night of the alleged triple ko game with Honinbo Sansa.
This what might called the conservative usage is also common in shogi (there is a proverb that the centre square is the Tennouzan).
In this usage in go, Tennozan nearly always therefore refers to not just a big strategic point but to a high, commanding one. A fourth-line extension might call forth its use whereas the same extension on the third line probably would not. A cap or a moyo-related move certainly fits the bill but it must dominate in some way. It's probably true to say also that there must be some expectation of a fight around this play - a move just enclosing a large territory or moyo, even on the fourth line or higher, should never elicit this term.