pi·lus (p??l?s)n. pl. pi·li (-l??) A hair or hairlike structure, especially a proteinaceous structure projecting from the surface of a bacterium that is smaller than a flagellum and functions in DNA transfer during conjugation and, usually with other such structures, in adhesion.[Latin.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.pi?lus (?pa? l?s) n., pl. -li (-l?). a hair or hairlike structure. [1955?60;