n. 1. An opening, such as a hole, gap, or slit. 2. 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.] aper·tur′al adj. |
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