velum

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Biology A covering or partition of thin membranous tissue, such as the veil of a mushroom, a rim of tissue around the margin of the bell of certain cnidarians, or a membrane of the brain.
• noun Anatomy The soft palate.
• noun Zoology A ciliated swimming and feeding organ that develops in certain larval stages of most gastropod and bivalve mollusks.

from The Century Dictionary.

• noun [capitalized] One of the subdivisions of the constellation Argo Navis. See Argo, 2.
• noun Same as velarium, 1.
• noun In botany: In Hymenomycetes, a special membranous envelop which incloses for a time the whole or a part of the sporophore.
• noun In Isoëtes, the outgrown membranous margin of the fovea. Also called involucrum.
• noun In Mollusca, the highly characteristic ciliated formation of the embryo, which serves as an organ of locomotion in that stage when the embryo is called a veliger. It is usually soon lost, but in some cases is permanently retained in a modified form. See cuts under veliger.
• noun In Hydrozoa, a kind of flap or circular free edge which projects inward around the margin of the disk of many hydrozoans, as those which are bell-shaped or conical, and which from its presence are called craspedote; a velarium.
• noun In Infusoria, a delicate veil-like membrane bordering the mouth in such forms as Cyclidium and Pleuronema.
• noun In sponges, one of the transverse diaphragms or partitions which constrict the lumen of an incurrent or excurrent canal.
• noun In Rotifera, the trochal disk. See cuts under trochal, Rotifer, and Rotifera.
• noun In entomology, a membrane attached to the inner side of the cubital spur in certain bees.
• noun In anatomy, a veil, or a part likened to a veil.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Anat.) A curtain or covering; -- applied to various membranous partitions, especially to the soft palate. See under Palate.
• noun See Veil, n., 3 (b).
• noun A thin membrane surrounding the sporocarps of quillworts Isoetes).
• noun (Zoöl.) A veil-like organ or part.
• noun The circular membrane that partially incloses the space beneath the umbrella of hydroid medusæ.
• noun A delicate funnel-like membrane around the flagellum of certain Infusoria. See Illust. a of Protozoa.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A thin membrane, resembling a veil, such as:
• noun meteorology An accessory cloud resembling a veil extending over a large distance. Normally associated with cumulus and cumulonimbus.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a membranous covering attached to the immature fruiting body of certain mushrooms
• noun a muscular flap that closes off the nasopharynx during swallowing or speaking