A somewhat literary or elevated word for the eye; everyday Japanese normally uses 目. Also appears naturally in expressions such as 寝ぼけまなこ.
He opened his eyes wide in surprise.
The baby gazed at its mother with big eyes.
彼は寝ぼけまなこで台所に入ってきた。
He came into the kitchen with sleepy eyes.
pupil and dark iris
Archaic use referring specifically to the dark central part of the eye, rather than the eye as a whole.
古い文では、「まなこ」が黒目の部分を指すことがある。
In old texts, manako can refer to the dark part of the eye.
insight; perceptivity; power of observation
Archaic figurative use for the ability to see through things or perceive their nature.
古語では、「まなこ」が物事を見抜く力を表すこともある。
In classical usage, manako can also mean the ability to see through things.
Archaic extension from the physical eye to one's gaze or the range of what one can see.
古い文献では、「まなこ」が視線や視界を意味する場合がある。
In old writings, manako may mean one's gaze or field of vision.
Archaic figurative use for the central or essential part of something.
古語の注釈では、「まなこ」を物事の中心や本質と説明することがある。
In notes on classical language, manako is sometimes explained as the center or essence of something.
砂が眼に入って痛かった。
Sand got into my eye and it hurt.
瞳 usually refers to the pupil or poetically to the eyes themselves, with a softer, more romantic tone than 眼（まなこ）, which is simply a literary word for the eye.
眼球 is the clinical, anatomical term for the eyeball, whereas 眼（まなこ）is a literary, non-technical word for the eye.
bloodshot eyes · frenzy; desperation
use of multiple simple paper masks to show emotions in Edo-period theater · simple paper mask
goggle-eyes; round, sunken eyes; greedy look
sleepy eyes; drowsy look
goggle eyes; big round eyes
Attested kanji spelling for the reading まなこ. The same character 眼 has other readings in other words, so context or furigana may be needed.
Kana spelling is a practical way to make the native reading clear, especially because 眼 can be read in other ways.
まなこ is a native Japanese word. The kanji 眼 is conventionally associated with the meaning 'eye'; the exact historical derivation of the word is uncertain here.