Pili are hair-like structures that are present on the surface of some bacteria and other types of microorganisms. There are several synonyms for this word, including fimbriae, flagella, and cilia. Fimbriae are short, hair-like projections that help bacteria adhere to surfaces and other cells. Flagella are long, whip-like structures that bacteria use for propulsion. Cilia are short, hair-like projections that line many cells in the body and help move fluids and particles. Each of these structures is unique and serves a specific function, but they are all important for the survival and function of microorganisms in their environments.