A kind of skull cap of felt.

The expanded upper portion of many of the fungi. See Mushroom.

An accessory cloud of small horizontal extent, often cirriform, in the form of a cap, hood, or scarf, which occurs above or attached to the top of a cumuliform cloud that often pierces it. Several pileus clouds fairly often are observed above each other. Pileus occurs principally with cumulus and cumulonimbus.

a cap as in a mushroom cap