pileus

noun, plural pi·le·i [pahy-lee-ahy, pil-ee-ahy] /ˈpaɪ liˌaɪ, ˈpɪl iˌaɪ/ for 1, 2, 4, pi·le·us for 3.

1. Mycology. the horizontal portion of a mushroom, bearing gills, tubes, etc., on its underside; a cap.
2. Zoology.
   1. the umbrella or bell of a jellyfish.
   2. pileum.
3. Also called cap cloud, scarf cloud. Meteorology. a small, thin cloud just above or attached to a growing cumulus cloud.
4. a felt skullcap worn by the ancient Romans and Greeks.

noun plural -lei (-lɪˌaɪ)

1. the upper cap-shaped part of a mushroom or similar spore-producing body

Plural pilei (pī′lē-ī′)

1. The umbrellalike fruiting structure forming the top of a fleshy fungus. It is supported by the stipe. The cap of a mushroom is a pileus.