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Synonyms
ap·er·ture (ăṕər-chər)

[Middle English, from Latin apertūra, from apertus past participle of aperīre, to open.]

noun 

  1. An opening, such as a hole, gap, or slit.
    1. A usually adjustable opening in an optical instrument, such as a camera or telescope, that limits the amount of light passing through a lens or onto a mirror.
    2. The diameter of such an opening, often expressed as an f-number.
    3. The diameter of the objective of a telescope.

derivatives

aṕer·tuŕal
adjective