Pronunciation: (pī'lē-us, pil'ē-),[key] —n., —pl.pi•le•iPronunciation: (pī'lē-ī", pil'ē-ī")[key]for 1, 2, 4,pi•le•usfor 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.
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.