The left eye (the eye on the left side). Used often in medical or formal contexts; in everyday speech 左目 is more common.
医師は患者の左眼の視力低下を確認した。
The doctor confirmed decreased vision in the patient's left eye.
左眼に異物が入ったので、流水で洗い流した。
Something got into my left eye, so I rinsed it with running water.
彼は試合中に左眼を負傷して病院に運ばれた。
He was injured in the left eye during the match and was taken to the hospital.