How would you define "Fluent" level in English?

Most of us heard about such levels as Beginner, Intermediate, Upper Intermediate, Advanced.

As I understand "Fluent" is the highest level when describing someone's English.

But how would you describe it? Some people think that "Fluent" is just when you speak very good (relative to other non-native speakers). But I think this is not enough. I think that you can say that you speak fluently when it is hard for native speakers to understand (based on your speech, accent and grammar) that English is not your native language. And if a native speaker can quickly (matter of seconds, or a minute) understand that your are a foreigner - you don't have a fluent level.

What do you think?

For an answer to this question, I will refer you to Jack Seward, who covers this topic specifically in his book Japanese in Action. Although he is talking about Japanese, the same things are true of any language, including English:

To be accurately judged fluent in [a language], I believe a [non-native speaker] should have the following qualifications:

1. He should be able to conduct all his daily affairs (business, visits to the doctor, TV-ing, bar-hopping, lovemaking, etc.) completely in [the language] without strain.
2. His accent may not be perfect, but it should occasion no confusion or merriment among his listeners.
3. He should be able to read [publications in the language] (newspapers, weekly magazines, and letters in the [cursive] ... style), with only an occasional reference to a dictionary.

He goes on to propose that a test for fluency in the language should require the test taker to:

1. Translate a newspaper article
2. Speak in [the language] on the telephone, as a test of accent.
3. Write a letter in [the language].
4. Interpret a taped conversation between two [native speakers of the language].
5. Comprehend a newscast.
6. Identify five major dialects.
7. Read a letter written in [cursive handwriting].
8. [Not applicable]
9. Give the meanings of one hundred technical words or phrases ... from the fields of medicine, law, economy, science, and the arts ... [which are readily understood by the average native-speaker who is a college graduate].
10. Walk down the street and [read and interpret] the first twenty signs to be sighted.
11. Give a ten-minute, impromptu talk about an everyday topic of conversation (sports, politics, travel, traffic, etc.), the topic to be selected [at random].

Strictly speaking 'fluent' in relation to speech means only that it flows smoothly and easily. There is no implication that you shouldn't be able to detect the non-native status of the speaker, and I think this is a perhaps too high a standard.

If someone speaks English as well as the average Englishman, but has a slight accent that betrays his foreign origins, does that mean he isn't fluent? I'd consider someone fluent if they didn't need to pause to think of a word any more than a native.

I would propose that a "fluent" speaker is one who can learn the language in the language.

This of course needs some clarification, as newborns are not fluent in their regional language even though they primarily learn to speak in their regional language. I would argue that everyone learns their first language in info-graphic association (or equivalent for those with disabilities).