pileus

plural pilei

1. biologyThe 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.

   1903, George Francis Atkinson, chapter VII, in Studies of American Fungi. Mushrooms, Edible, Poisonous, etc., 2nd edition, New York: Henry Holt

   1 more example

   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.

   1994, Barry H. Rumack, David G. Spoerke, Handbook of Mushroom Poisoning: Diagnosis and Treatment, CRC Press, →ISBN, page 75

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

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

   2011, Gavin Pretor-Pinney, The Cloud Collector's Handbook, Hachette UK, →ISBN

   Synonyms: cap cloud, scarf cloud

4. Ancient RomehistoricalA 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.

   1859, William Smith, editor, Dictionary of Greek and Roman Antiquities, page 920

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

   Synonyms: pileum, crown