Her pupils were jet black and deep, like a starry night.
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瞳 alone is slightly poetic or literary. In everyday usage, 瞳孔 (tóng kǒng) is the standard medical/everyday term for 'pupil.' 眼瞳 is a poetic variant meaning the same thing.
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瞳 uses the eye radical 目 on the left, and 童 (child) on the right — the pupil is the 'child' (tiny dark figure) reflected in the eye, which is exactly where the word's etymology comes from in many cultures.
