The pupil/eye (the visible central part of the eye); often used to refer to one's eyes, especially when describing their appearance.
彼女は大きな瞳をしている。
She has large eyes (big pupils).
暗い場所では瞳が大きくなる。
In dark places the pupils dilate.
その絵の瞳の描き方がとてもリアルだ。
The way the eyes are drawn in that painting is very realistic.
One's eyes (used poetically or expressively); often refers to a person's gaze or the expressive quality of their eyes.
彼は優しい瞳で子どもを見つめた。
He gazed at the child with gentle eyes.
瞳に涙を浮かべながら話す彼女は悲しそうだった。
She looked sad as she spoke with tears in her eyes.
瞳の色が変わったように見えた。
It seemed as if the color of her eyes had changed.