@rinkworks, doing nothing is sometimes good for the tempo. One bad buy can prevent you from drawing your entire deck.
It's not a point I care to press too hard, since the concept seems to apply only fuzzily to Dominion. But I would say that doing nothing
always loses a tempo. By contrast, buying a card your deck doesn't need or want
also loses a tempo AND worsens your future prospects. The equivalent in chess would be making a
ineffectual move that drops the initiative and making a
bad move that drops the initiative. In both cases, you've lost a tempo; in the latter, you've additionally worsened your future prospects. (Which may mean you'll be forced to lose further tempi [the plural form] in future turns, although that's not yet clear. Tempo only refers to the present, so you don't gain or lose tempo until the moment you gain or lose it.)
To me, tempo in Dominion would be related to how many turns --
in relation to your opponent -- you're taking to build your engine to the point where you can start buying green -- and, afterwards, the rate at which you are buying that green. Doing nothing on a turn means you're skipping one of your chances to acquire the next card your deck needs to stay ahead of your opponent. The same is true of buying a Silver your deck doesn't want just to do
something. Either way, you've lost a tempo.
Sometimes in Dominion, shuffle luck forces you to lose a tempo. There's just nothing you can do about it. That's true in Chess as well. A smart player may maneuver his opponent into
having to lose a tempo. The point is, tempo is really just a count of how many moves ahead or behind your opponent you are.
I agree with you that during the greening stage, it's about the rate at which you acquire VPs. But specifically it's the rate at which you acquire VPs
in relation to your opponent, and moreover you can only measure it in discrete turn numbers. You can't say "well, my opponent has a fast tempo, but mine is 10% faster, so I overtook him." You'd say, "It's a fast game, but I had the better deck, so I was able to recover the tempo."
At least, that's how you'd use "tempo" by the strictest definition. If people here want to twist the definition a bit to refer to a concept that is better suited to the game of Dominion, I don't see any reason to quibble with that. But without people explicitly agreeing to a standard ahead of time, by no means is it clear or self-evident what the term means to any given reader unless it's used by its strict definition.