镜 (jìng) is a bound form (不成词语素) that cannot stand alone as a word. It appears in compound words related to mirrors, lenses, and glass surfaces, such as 镜子 (mirror), 眼镜 (glasses), 镜头 (lens/camera lens), and 镜片 (lens/glass piece). Learners should always use it within a compound—never as an independent noun.