A small or narrow opening, gap, slit, or hole.
The diameter of such a hole which restricts the width of the lightpath through the whole system. For a telescope, this is the diameter of the objective lens.
A hole which restricts the diameter of the lightpath through one plane in an optical system.
The (typically) large-diameter antenna used for receiving and transmitting radio frequency energy containing the data used in communication satellites, especially in the geostationary belt. For a comsat, this is typically a large reflective dish antenna; sometimes called an array.
The maximum angle between the two generatrices.
A funnel-shaped expansion of the vas deferens or oviduct of insects.
A chamber in the mammalian kidney through which urine passes.
The outermost whorl of flower parts, comprising the sepals, which covers and protects the petals as they develop.
The crown containing the viscera of crinoids and similar echinoderms, entoprocts, and the polyps of some cnidarians.
A flattened cap of neuropil in the brain of insects.