pileus

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

• noun Botany The umbrellalike fruiting structure forming the top of a stalked fleshy fungus, such as a mushroom; the cap.
• noun A brimless round skullcap worn by ancient Romans.

from The Century Dictionary.

• noun Among the ancient Romans, a conical cap or hat of felt; a cap or skullcap.
• noun In botany, the expanded cap-like or umbrella-like summit of the stipe, bearing the hymenium, in hymenomycetous fungi: same as cap, 2 . See cuts under Agaricus and Fungi.
• noun In ornithology, same as pileum.
• noun [capitalized] [NL.] A genus of echinoderms.
• noun A nipple-shield.
• noun The disk or umbrella of a jellyfish.

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

• noun (Rom. Antiq.) A kind of skull cap of felt.
• noun (Bot.) The expanded upper portion of many of the fungi. See Mushroom.
• noun (Zoöl.) The top of the head of a bird, from the bill to the nape.

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

• noun the cap of a mushroom.
• noun the bell of a jellyfish
• noun meteorology a small thin cloud attached to a cumulus cloud. A cap cloud
• noun a conical felt hat worn in ancient Rome and Greece
• noun The top of the head of a bird, from the bill to the nape.

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

• noun a fruiting structure resembling an umbrella or a cone that forms the top of a stalked fleshy fungus such as a mushroom