Spirillum \Spi*ril"lum\, n. [NL., dim. of L. spira a coil.] (Biol.) A genus of common motile micro["o]rganisms ({Spirobacteria}) having the form of spiral-shaped filaments. One species is said to be the cause of relapsing fever. Spirobacteria \Spi`ro*bac*te"ri*a\, n. pl.; sing. {Spirobacterium}. [NL. See 4th {Spire}, and {Bacterium}.] (Biol.) See the Note under {Microbacteria}.