C1 refers to a specific level in the Common European Framework of Reference for Languages (CEFR), which is a guideline used to describe language proficiency. At the C1 level, individuals can understand a wide range of demanding texts and express themselves fluently and spontaneously without much obvious searching for expressions. This level indicates a high degree of language mastery. People at the C1 level can use the language effectively for social, academic, and professional purposes. They can produce clear, well-structured text on complex subjects and demonstrate a good command of idiomatic expressions and colloquialisms.