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.
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.
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.
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.
In old writings, manako may mean one's gaze or field of vision.
Archaic figurative use for the central or essential part of something.
In notes on classical language, manako is sometimes explained as the center or essence of something.
Sand got into my eye and it hurt.
眼球 is the clinical, anatomical term for the eyeball, whereas 眼（まなこ）is a literary, non-technical word for the eye.
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.