She is fluent in English. I am a native speaker of German. I am not sure he speaks Pashto well.
I am not sure what any of the sentences above mean. The other term that gets thrown around in the circles in which I move – often – is language proficiency. It is a useful concept, but what does it mean?
In Applied Linguistics, proficiency is operationalized – for research, teaching, and testing and meaning getting ready to be measured transparently – as a collective variable that consists of complexity, accuracy, and fluency as are evident in spken or written texts. Each – complexity, accuracy, and fluency – are also collective variables, which means each is made up of a relatively stable configuration of smaller variables. The make-up of complexity is diversity (a larger range of vocabulary (lexical) and a larger inventory of linguistic constructions (syntactic) are more complex) and sophistication (longer words with a more elaborate morphological structure (lexical) and longer sentences with additional adjectival phrases, modifiers, and sub-clauses (syntactic) are more complex). Linguistic constructions, sentences, and texts that deviate less often and less significantly from an expected norm are more accurate. Uttered texts that contain more linguistic constructions, for example words, per time unit or task unit and that are more coherent and cohesive are perceived as more fluent by listeners and readers. Increasing complexity and fluency of learner texts normally correlate; for example, learners with a larger accessible vocabulary tend to be more fluent. On the other hand, there are trade-off effects between accuracy and complexity; when students focus on producing more complex and longer texts, they tend to make more mistakes proportionately.