Good question, and which I think will attract quite different answers. I would argue that 'intermediate' is the level at which you can use the language well enough to be understood, to follow what is being said by someone speaking clearly in a standard accent, to hold a conversation, to be able to manage fairly well in most regular situations.
It does not imply that you don't make mistakes in grammar, vocabulary, or construction, or that you have a good knowledge of colloquial usage; I would expect this from someone claiming to be advanced [C2].
Although I have been learning French for many years, I do not believe that I have reached a level beyond intermediate. I also suspect that most of those who claim to be advanced are also, in fact, intermediate.
Only by testing a person could you verify his or her actual level, and this is what IELTS and similar tests attempt to do.
So you've can read Hangul and knows the basic grammar.
You can read and write sentences.
You got one thing to improve now.
Listening skill.
I think there is no clear boundary since everyone has different definition.
Personally for me it is a range of level from you are doing simple conversation regarding most of the subject (even if you don't know the subject you can ask any question regarding the subject at the moment and continue the conversation then I would say you are intermediate) to all the way to the level anyone can understand what you are saying and they interact with you without any major problem except minor mistake such as slightly wrong grammar or the words not used often in actual conversation.
If you go to advanced level I would say this is a level you can't tell the person is native or not unless you hear the accent or not native like way of speaking, expression etc. I would say when you are in advanced level you can go head on head with native regarding any debate, writing a argument,etc.
That's my personal standard regarding speaking language but generally if I see anyone speaking Korean and can engage any conversation when I consciously/unconsciously speak slightly slow tempo then it is in intermediate category. If end of the conversation I didn't even realize that I spoke anything without any considering the person speaks Korean as second language then definitely advanced level.
