What determines A Legendary? (Pokemon Diamond)?

Question

A couple of my friends and I can't tell, what makes a pokemon legendary, Is it the way you meet it? The moves it learns? or the fact that you can only get 1 legitly (and without trading) or that you cannot bred them

For Example

Dialga is a legendary, you meet it at an event, you only can get 1 in the game, and it learns the Unique move, roar of time, and it cannot be bred

Spiritomb is NOT a legendary, you do meet it at a minor event, you can get multiple, and it learns no unique moves, and it can be bred

What about rotom, minor event, you can only get 1, no unique moves, and cannot be bred,

Personally, I believe that for it to be a legendary, you are only allowed to get 1, and it cannot be bred?

share your thoughts, person who clears it up for us gets the points

Answer 1

1. some legendary pokemon are hard to beat

2. most are hard to catch

3. they barely appear in the series

4. in movies they are the most powerful but in the game depends on how you train it.

Answer 2

I really just think it's the "lore" that determines a legendary status, nothing related to game mechanics.