1. An opening, such as a hole, gap, or slit.
a. 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.
b. The diameter of such an opening, often expressed as an f-number.
c. The diameter of the objective of a telescope.
[Middle English, from Latin apertūra, from apertus, past participle of aperīre, to open; see wer-4 in the Appendix of Indo-European roots.]
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