The quality of state of being proficient; advance in the acquisition of any art, science, or knowledge; progression in knowledge; improvement; adeptness; as, to acquire proficiency in music.
Mastery or ability to do something at grade level. California students receive scores on the California Standards Tests (CST) that range from "far below basic" to "advanced." The state goal is for all students to score at "proficient" or "advanced." (Ed Source)
Proficiency consists of the learner's knowledge of the target language; it can be considered synonymous with 'competence'. 'Proficiency' can be viewed as linguistic competence or communicative competence. L2 proficiency is usually measured in relation to a native speaker's proficiency.