What is Fluency?

What is language fluency?

Google Define gives two definitions:

The quality or condition of being fluent, in particular. The ability to speak or write a foreign language easily and accurately.

The first definition is not very relevant (nor very good), but the second is better.

How can you measure this? Is it measurable?

I have seen (and taught myself) people who I felt are very fluent, but once I went to teach them, because the vocabulary and grammar was new, reverted to what they knew and struggled. I think it is important to continue to try to expand on your knowledge as a language speaker, foreign or even mother-tongue, no matter your level. I think there is always something to be learned or improved upon.

The purpose of language is communication, so with that in mind, what is the best way to achieve the second definition (The ability to speak or write a foreign language easily and accurately) as a teacher?

From my own experience, fluency is extremely subjective. Someone who might appear to be "fluent", can easily flounder when put into an unfamiliar situation.

I am still trying to figure out the best way to teach young students vocabulary as I feel that is the best way to become fluent.

Can fluency be measured? No way. But we can measure learning.