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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- in Indo-European roots.]

ap′er·tur′al adj.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.


(ˈæpəˌtjʊərəl; əˈpɜːtjərəl)
relating to aperture or an aperture
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
References in periodicals archive ?
Kemp and Berthess (1984) found that differences in growth rate induced shell shape variation in Littorina littorea; Appleton and Palmer (1988) showed that Thais lamellosa developed differential aperture characteristics when exposed to waterborne stimuli released by predatory crabs and damaged conspecifics; Lindberg and Pearse (1990) demonstrated that the color and apertural morphology of the limpet Lottia asmi changed with different substrates and diets.
Shell: Medium sized, depressed-discoidal to planorboid; spire very low (H:D=0.460.62), often with only embryonic whorls projecting in apertural view; final part of last adult whorl descending prior to aperture, but not steeply so; whorls more or less evenly rounded, suture indented; umbilicus very wide, its margin evenly rounded, underside of embryonic whorls clearly visible.
testudinaria living on crabs and turtles, respectively, small individuals live on large ones near the aperture and act as males (often called "apertural males"; Crisp, 1983; Zardus and Hadfield, 2004).
SEM: tectum very perforate, apertural membrane scabrate (116 of Figs.
Shell ornamentation, an increase in shell size or thickness, and complex apertural morphology may have evolved in response to changes in the type and strength of predation (Vermeij 1977).
This study focuses on three characters [ILLUSTRATION FOR FIGURE 1 OMITTED]: apertural inclination (elevation angle of Vermeij 1971; InAn of Schindel 1990), sinus width, and shell torque (standardized torsion of Okamoto 1988; [Gamma][prime] of Ackerly 1989).
Shell length and diameter measurements were made with the shell held in apertural view with the axis of coiling vertical.
SEM images are displayed in vertical columns, one column per genus, and characterise four views: polar, apertural, equatorial, and fractured sections, sometimes taken from different species in a genus, often from different collections of the same species, in order to document as much variation and/or similarity within groups as possible.
As such, these apertural plications cannot affect the function of the columellar muscle, so I consider Oliva sayana to be without folds.
Like many marine gastropods, the Tanganyikan gastropods are often heavily calcified and many species display coarse noded ribbing, spines, apertural lip thickening and repair scars [ILLUSTRATION FOR FIGURE 1 OMITTED].
Dimensions: Greatest length 9.0 mm, greatest diameter 8.0 mm, but size at maturity evidently variable; some specimens of length ca 7.0 mm possess mature apertural dentition.