I'd say that, particularly in modern genre terms, "Metroidvania" refers to a game involving gathering abilities that unlock new areas, with either mandatory or optional-for-bonuses backtracking to previously blocked paths. (This is distinct from just flipping switches or finding keys -- you have to find things that change your character's moveset, and the new capabilities are what let you pass through.)
Non-linearity and ability-acquisition are the only essential elements. There's often platforming, a certain isolation-vibe, and a large single map, but those aren't mandatory to get the label.
Admittedly, that definition as given leaves most Zelda games qualifying. You go around gain abilities, there are often optional rewards for backtracking and using those abilities. (BOTW is a pretty major exception here, and so are the NES zeldas).
One notable difference is that there is rarely benefit to backtracking into previously completed dungeons. Even for 100% it is often possible to get everything with the first pass (without even using glitches or unintended interactions), which is not like most metroidvanias. However, most Zelda games but the games do tend to favor overwold backtracking for optional rewards.
Non-linearity and ability-acquisition are the only essential elements. There's often platforming, a certain isolation-vibe, and a large single map, but those aren't mandatory to get the label.