A hair, or a hair-like structure. Most often it refers to thin outgrowths on bacteria, usually used for attachment or conjunction. When used for attachment, they may also be known as a fimbria (pl. fimbriae). When used in conjunction, it is known as the F pilus (or sex pilus), and is used for transferring DNA.

Pronounced pee-lus. The plural is pili, and the prefix pili- refers to hairs. Pilus is Latin; in Latin, it can also mean 'a trifle', or refer to a javelin. Thus a Primus Pilus (first javelin) was the chief centurion in a legion.