velum

Short definition from Wiktionary

Noun

1. 1
   A thin membrane resembling a veil or curtain: The soft palate.
2. 2
   A thin membrane resembling a veil or curtain: A thin membrane partially covering the cluster of sporangia near the leaf base in quillworts and their extinct relatives.
3. 3
   A thin membrane resembling a veil or curtain: A veil-like membrane of immature mushrooms extending from the margin of the cap to the stem and is torn by growth, to reveal the gills.
4. 4
   A thin membrane resembling a veil or curtain: A locomotory and feeding organ provided with cilia found in the larval stage of bivalves.
5. 5
   A thin membrane resembling a veil or curtain: An annular membrane, typically bordering a cavity, especially in certain molluscs, medusae, and other invertebrates.
6. 6
   A thin membrane resembling a veil or curtain: A delicate membrane found on certain protists.
7. 7
   An accessory cloud resembling a veil extending over a large distance; normally associated with cumulus and cumulonimbus.
8. 8
   a thin coating or film of mucus or phlegm.