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A pilus (Latin for 'hair'; pl.: pili) is a hair-like cell-surface appendage found on many bacteria and archaea. The terms pilus and fimbria (Latin for 'fringe'; plural: fimbriae) can be used interchangeably, although some researchers reserve the term pilus for the appendage required for bacterial conjugation. All conjugative pili are primarily composed of pilin – fibrous proteins, which are oligomeric.
Dozens of these structures can exist on the bacterial and archaeal surface. Some bacteria, viruses or bacteriophages attach to receptors on pili at the start of their reproductive cycle.