Started to play Hollow Knight...

Nah, that is ok -- I am actually enjoying not having perfect directions all the time... well, at least so far, because I have not yet felt totally lost... But if I ever get frustrated not knowing where to go next, I will surely try that link, thank you! :D

I found out about the map inside the game, and I thought it was a neat idea.

lol didn't know you could zoom it for a min. was like damn.

It's worth noting that it's the "backtracking" (ie, unlocking new routes or interactions in previously visited areas) that defines Metroidvanias more than the platforming component. Hence, much like roguelites only sharing a portion of the traits of roguelikes (and typically emphasizing only the procedural generation or repeated-play elements), you do get "Metroidvania-lites" which primarily base in that core aspect alone.
Of course, the concept of backtracking unlocks was already widespread well before Castlevania and Metroid popularized it (as off-hand examples, Commander Keen games and Zelda games), making it rather difficult to maintain a broader association for the mechanic. Hence, while roguelites (excepting where they've formed their own subgenre, such as Diablo-likes) typically fall under roguelikes in general reference, Metroidvania tends to be a bit more rigid in specifically referring to side-scrolling platformers [a distinction that's similar to how proper roguelikes are all tile-based and turn-based, with those being two of the traits rogue-lites most often differ on].
This is of course notable because most modern Castlevania and Metroid games are 3D platformers or action-adventure games, making modern associations of the term either very rigid, or very loose in application (as the individual prefers).

In short, it's hard to really know off-hand what someone means when they state that a game is a Metroidvania, except for the reliable fact that the game involves some sort of progression-locking mechanism, requiring some degree of backtracking as the game progresses. Put another way, Metroidvania can't necessarily be used as a term for all games which involve backtracking and unlocks, but any game which is referred to as a Metroidvania will (regardless of its other characteristics) certainly have backtracking emphasized.

Also, if you're ever curious about a term, Wikipedia tends to be extremely accurate and detailed about gaming terminology of any sort (though it does tend to be unreliable on subjective elements, such as to what degree a specific aspect of a genre should be weighted). You can usually get a good idea of what a term refers to just from the opening paragraph(s) of any wikipedia article. :)
..And it's my opinion that it should be mandatory for all Steam publishers to view the wiki article on Co-Op before being allowed to post up any game, so that we don't keep getting Team PvP games listed under the tag. -.-;

To add to Sooth's description, Metroidvanias do not necessarily involve lots of backtracking. What they do involve is gradually gaining abilities that open up more of the world to explore, so that going back with new abilities to areas you previously went through now lets you access parts of those areas that you couldn't before. There's a constant sense of progress and exploration and unlocking new tools that let you explore new areas and more parts of old areas.

Backtracking is inherent to Metroidvanias, but it need not necessarily feel like there is a lot of it. For example, if you look at one of the classics and standout examples - Castlevania: Symphony of the Night - there are many things that greatly speed up navigation. From being able to teleport around the map via the fast travel system, to being able to kill early enemies in a single hit, to gaining abilities that let you run through or avoid enemies, the game is designed such that initial exploration of an area is slow while repeat visits to that area are fast and efficient. It's very different from, say, a JRPG that features a lot of backtracking where you're constantly being interrupted and slowed down by random encounters. In a well designed Metroidvania, the backtracking should never feel tedious.

No apology needed -- I did spend 90% of that post misrepresenting my real feelings, for dramatic effect. :D

Cheers!

Oh, ok -- I have seen this guy. :D