Well, I think that tempo is kind of a combination of the 2 things you are you are talking about. I would call the first thing click efficiency and is only tangentially related to the other which I would call something like "threat". Getting the most out of each click certainly helps you maintain your threat, but it doesn't necessarily mean you are dictating the pace of the game. Example: Late in the game both sides are on match point. The runner is making 8 credits a turn compared to the corps 3, but it costs 15 to get into R&D and 10 to get into HQ. The corp has BL in hand (no Clot in the runner's deck for the sake of the example). Even though the runner has much better click efficiency, the cost of meaningful accesses still swings the threat in favor of the corp. At least until the runner plays an RDI.
By the same token there are situations where you can dictate the course of the game by clicking 3 times for credits and making 1 run provided that the run is threatening enough. The corp could be gaining 6 credits a turn to your three, but if you can still get those meaningful accesses cheaply, you still have the threat. Example: Late game both sides are looking for their last agenda and the runner has 3 RDIs and can just make enough money to see those cards every other turn. Runner has the threat. At least until the corp drops a Tollbooth or a Caprice on R&D.
So click efficiency is about getting the most out of your clicks. Threat is based more on runner's ability to access and the corps ability to score. One helps to maintain the other, but you don't necessarily need to have the better click economy to maintain threat.
Both examples that I gave above, threat can easily be reversed by playing certain cards, and the better click efficiency of the opponent makes those plays very conceivable. Neither side in those examples has what I would call tempo. Tempo is the ability to use your threat to force your opponent down a path that yields lower click efficiency while maintaining good click efficiency for yourself. This relative advantage in click efficiency helps to extend the duration of your threat. This is the snowball effect that a lot of people have mentioned. I think that the key to tempo is that your threat negatively effects your opponent's click efficiency. They are forced to scramble to correct the situation. If both sides have relatively strong click efficiency and are just throwing hay-makers at each other, I wouldn't say either side has a particular tempo advantage because things could change quite quickly.
With that in mind, I would say that a strong tempo play does 3 things: 1. It creates a strong threat. 2. It requires the opponent to make a change to the board to stop that threat or risk losing and 3. They are unable to fully neutralize the threat on their next turn and are forced to scramble (normally by clicking for credits or cards). Less strong (but in no way weak) tempo plays can be addressed the turn after they are made, but leave the other side too resource poor to develop their own threat. In Hearthstone this is the equivalent of forcing your opponent to trade his board plus all his mana for a couple of spells in hand for your board, which allows you to put the first creature down.
Edit: Clarified my last paragraph and added the Hearthstone analogy.