Eternal salvation almost certainly refers to salvation in the final judgement. There is a kind of temporal salvation I think. If you look at how Jesus uses the term salvation, he uses it two ways: (1) for final salvation, (2) for people who change from God's enemies to his friends. (e.g. Luke 19:9).
The commentary I checked notes that the phrase "eternal salvation" occurs nowhere but Hebrews. (They're apparently not counting Mark 16:8, as it's not in the original text.) They believe it may be a reference to Is 45:17, where it does appear to be salvation that starts now and continues forever. But enough other places in the NT talk about the possibility of falling away that I wouldn't take this passage as the only answer, particularly since Hebrews itself sees the possibility of falling away (10:26).
It's possible, of course, to use the typical Calvinist approach, and say that people who fall away weren't saved in the first place. I guess it's possible that the author of Hebrews has that in mind, but I don't see any sign of it. The difficulty is that 10:29 says that these people who fall away were sanctified.
I'm willing to accept the Reformed perspective in a certain sense. When you look at a life from the end, or from God's perspective, you may see that something that looked like salvation was just a temporary episode. So in terms of God's plan, they were never elect, if you accept that concept. But for those of us who aren't God and are in the middle of history, we can't tell the difference. Not even the person can.
It's really difficult to use logic to describe the Christian life. The Kingdom has both a present and a future. We are genuinely in the Kingdom now, even though it won't be fully visible until the end. John uses eternal life the same way: we can enter it now, and it continues.
Still, it's pretty clear that some people don't continue. Did they enter eternal life and then leave it? Or were they never really in it? I don't think either answer is fully satisfying. The whole concept of eternal life doesn't allow for leaving. But saying people who are, as far as anyone including them can tell, living in the Kingdom really weren't in the Kingdom after all, seems kind of like a legal fiction. There may be no good answer to something that shouldn't happen in the first place (even though it does).
The one answer that I think is clearly wrong is that once someone does some specific thing (prays the sinners prayer or does something else to indicate that they're born again) they're permanently saved no matter what their life is like later.