/puy"lee euhs, pil"ee-/ , n. , pl. pilei /puy"lee uy', pil"ee uy'/ for 1, 2, 4, pileus for 3.
1. Mycol. the horizontal portion of a mushroom, bearing gills, tubes, etc., on its underside; a cap. See diag. under mushroom .
2. Zool.
a. the umbrella or bell of a jellyfish.
b. pileum.
3. Also called cap cloud, scarf cloud . Meteorol. a small, thin cloud just above or attached to a growing cumulus cloud.
4. a felt skullcap worn by the ancient Romans and Greeks.
[ 1750-60; pileus skullcap; akin to Gk pîlos felt, felt cap ]