What does pili mean?
Pili is the plural of the rare noun pilus, meaning a single hair or a hair-like structure. In biological and zoological writing, it may refer to the fine projections found on the bodies of insects, microorganisms, and other organisms, including structures used for attachment, sensation, movement, or protection. Because pilus itself is uncommon outside specialist contexts, ordinary speakers are more likely to use the everyday plural hairs. Pili is consequently most useful to readers of scientific and taxonomic descriptions, where its precise anatomical sense distinguishes individual hair-like structures from the general idea of hair. Its rarity also makes it potentially confusing with the unrelated British adjective pilly, meaning foolish or silly.
nounThe plural form of pilus, a rare noun used chiefly in biology and anatomy for a hair or a single hair-like structure.
- The plural of pilus, meaning one of the fine hairs or hair-like structures occurring on an animal, plant, microorganism, or other biological specimen.
"The microscope revealed several pili projecting from the surface of the organism."
"The organism uses its pili to attach itself to the surface."
"Short pili cover the insect's legs and help it maintain traction."
Pili is itself the standard plural of the rare noun pilus; it has no separate common plural form.
"Each of the animal's pili ended in a fine sensory tip."
In specialist biology, one tiny hair can be called a pilus—and many of them become pili.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does pili come from?
The word ultimately comes from Latin pīlus, meaning hair. English borrowed the singular pilus into specialized biological vocabulary, while its plural pili continued the Latin plural form. Pili was not adopted broadly into everyday English and survives chiefly in technical descriptions of hair-like biological structures.
How pili is actually used
This is an uncommon, chiefly technical or zoological form. It is not normally used in ordinary conversation to mean several hairs.
Easily confused with pili
Pili is the rare plural of pilus, whereas pilly is a British adjective meaning foolish or silly.