It's complicated. The meta is basically whatever is strongest or close to it in any given area of the game at some point. How close depends on who you ask. I also use it to refer to environments of top-level play and the times when those environments were common, for example I'll talk about using a hero "in the Gooric meta", meaning that I used that hero against Gooric back when Gooric was the top tank. The meta term doesn't really have one specific definition, so which heroes are considered "meta heroes" depends on who you ask.
Personally, I don't really think of heroes as being "meta heroes" or not because at the end of the day, every hero has some role in a team and I want to use that hero if and only if my team needs someone to fill that role.
For example, Hygd is a very powerful cleanser, so if I'm facing some ailments that need to be dealt with (for example they might have a Vernix and a Fafnir), I'd like to bring Hygd, but if there aren't any ailments, I'd prefer a healer that counters something else. Now you don't always get that choice because nobody has a complete sandbox account and even a sandbox account couldn't always fill every role on every team because there's only 5 slots.
This is where a type of hero that I really like comes in: Heroes that fill multiple important roles at once against some of the strongest defenses. For example, while he's probably not in many people's lists of "meta heroes", I really like Andvari because he removes fiends (good against vegetables), puts up a firestorm ward (useful against all sorts of hitters, most notably Thaffer) and gives a pretty big mana generation boost (which is always useful unless you're facing Haradea or something).
Those types of heroes might be what makes the difference between your team working and not working while a flashy new hero in their place would get destroyed because they aren't what you need in that scenario. And of course, a lot of heroes nowadays are so complex that they can be the flashy new thing and a great counter hero at once (for example Salome, who is really annoying to fight but also has a few countering aspects), but you also won't always have the best counter for everything unless you chase every other hero that gets released.
So the older counter heroes definitely also have their place in the "meta". All in all, don't concern yourself with what is or isn't a "meta hero", instead find out which teams make sense, what they work and don't work against and which heroes might be able to make them work against the defenses they struggle against.
