Darkness, mist, or dimness; in medicine, a condition causing dimness of vision or a dark spot in the visual field.
From Latin caligo (darkness, mist, dim-sightedness). Used in medical and literary English since at least the Renaissance for both literal darkness and metaphorical obscurity.
Romans used one word for both fog and blindness—caligo—because they understood something profound: obscurity can be physical, spiritual, or medical all at once.
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