pileus

noun

1. The cap of a mushroom.

   “The stem is easily separable from the pileus at its junction, in this respect being similar to Amanita, Amanitopsis, Lepiota and others.”

   “One must check the size and shape of the pileus as well as its color. Many weird and unexpected shapes are found in the mushroom kingdom.”

2. The bell of a jellyfish.
3. A small thin cloud attached to a cumulus cloud.

   “The most short-lived of all the accessory clouds, pileus is also the most beautiful.”

4. (Ancient-Rome, historical)A conical felt hat worn in Ancient Rome and Greece.

   “Among the Romans the cap of felt was the emblem of liberty. When a slave obtained his freedom he had his head shaved, and wore instead of his hair an undyed pileus.”

5. The top of the head of a bird, from the bill to the nape.