Eyeball
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Visual Organ
The word 'eyeball' refers specifically to the spherical part of the eye. The optometrist examined the health of her eyeballs.
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Not the Whole Eye
'Eyeball' does not include the eyelids, eyelashes, or eyebrows; it's just the visible sphere. His eyeball was irritated, but his eyelids were fine.
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Literally Visible
When speaking of 'eyeballs', it sometimes implies the colored part that is visible to others. Her green eyeballs stood out against her dark makeup.