pileus

noun

pi·​le·​us ˈpī-lē-əs 

plural pilei ˈpī-lē-ˌī 

1

: a pointed or close-fitting cap worn by ancient Romans

2

: the convex, concave, or flattened spore-bearing structure of some basidiomycetes that is attached superiorly to the stem and typically is expanded with gills or pores on the underside

called also cap

Examples of pileus in a Sentence

Recent Examples on the Web

Many historians trace it to the pileus, a cap worn by the formerly enslaved in ancient Rome to indicate that they had been freed. —Greg Daugherty, Smithsonian Magazine, 18 June 2024

In certain types of clouds (pileus, lenticular, and certain types of cirrus), brilliant colors can be seen. —Marshall Shepherd, Forbes, 22 Jan. 2023

But Tuesday’s storms had something a little special: a pileus. —Matthew Cappucci, Washington Post, 9 Aug. 2022

The species of the comb jellies that now cover the sand is Pleurobrachia pileus, and this is not the first time a massive group of them has washed up on a beach. —Mike Wehner, BGR, 5 June 2021

By the 14th century, pileus caps were becoming taller and more cylindrical, similar to a modern chef’s hat but shorter. —National Geographic, 11 June 2020

This style, the pileus rotundus, was adopted mainly by university students studying law, medicine, and the sciences. —National Geographic, 11 June 2020

Word History

Etymology

borrowed from Latin pilleus, pīleus, pilleum, pīleum "felt cap," of uncertain origin

Note: The word pilleus has been adduced as an example of the "littera-rule" in Latin (alternation between an initial syllable with long vowel followed by single consonant and the same with short vowel followed by double consonant, as in the word lītera/littera "letter"). However, the antiquity of pīleus with single l has been questioned, and the possible influence of Greek pîlos "felt" suggested. Etymologically, pilleus has been associated with both Greek pîlos and Latin pilus "hair" (see pile entry 5). A reconstructed stem *pils- has been proposed, that would tie together both Latin pilleus (from *pilsei̯os according to the Oxford Latin Dictionary) and the Greek word (allegedly from *pilsos). The *pils- hypothesis has been rejected, however, by C. Michiel Driessen ("Towards an Indo-European Term for 'felt'," Journal of Indo-European Studies, vol. 3, no. 1-2, spring-summer 2004, pp. 25-42). Driessen posits a proto-form *pis-lo- for Greek pîlos, from an Indo-European verbal base *pei̯s- "pound, crush" (see pestle entry 1), and regards Latin pilleus as of unknown origin. Other words for "felt" in Indo-European languages, which bear some phonetic similarity to pilleus and pîlos, are treated at the etymology and note at felt entry 1.

First Known Use

1663, in the meaning defined at sense 1

Medical Definition

pileus

noun

pi·​le·​us ˈpī-lē-əs 

plural pilei -lē-ˌī 

: the convex, concave, or flattened spore-bearing structure of some basidiomycetous fungi that is attached to the top of the stem and typically is expanded with gills or pores on the underside

called also cap