Webster's English Dictionary

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bacillus

ba.cil.lus \b*-'sil-*s\ \-'sil-.i-\ n or ba.cil.li [NL, fr. ML, small 
   staff, rod, dim. of L baculus staff, alter. of bacu] pl lum - more at 
   BACTERIUM 1: any of a genus (Bacillus) of aerobic rod-shaped bacteria 
   producing  endospores that do not thicken the rod and including many 
   saprophytes and some parasites (as B. anthracis of anthrax); broadly : a 
   straight rod-shaped bacterium 2: BACTERIUM; esp : a disease-producing 
   bacterium