The Collaborative International Dictionary of English v.0.48:

Bacterium \Bac*te"ri*um\ (b[a^]k*t[=e]"r[i^]*[u^]m), n.; pl.
   Bacteria (b[a^]k*t[=e]"r[i^]*[.a]). [NL., fr. Gr.
   bakth`rion, ba`ktron, a staff: cf. F. bact['e]rie.] (Biol.)
   A microscopic single-celled organism having no
   distinguishable nucleus, belonging to the kingdom Monera.
   Bacteria have varying shapes, usually taking the form of a
   jointed rodlike filament, or a small sphere, but also in
   certain cases having a branched form. Bacteria are destitute
   of chlorophyll, but in those members of the phylum Cyanophyta
   (the blue-green algae) other light-absorbing pigments are
   present. They are the smallest of microscopic organisms which
   have their own metabolic processes carried on within cell
   membranes, viruses being smaller but not capable of living
   freely. The bacteria are very widely diffused in nature, and
   multiply with marvelous rapidity, both by fission and by
   spores. Bacteria may require oxygen for their
   energy-producing metabolism, and these are called aerobes;
   or may multiply in the absence of oxygen, these forms being
   anaerobes. Certain species are active agents in
   fermentation, while others appear to be the cause of certain
   infectious diseases. The branch of science with studies
   bacteria is bacteriology, being a division of
   microbiology. See Bacillus.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

bacterium
    n 1: (microbiology) single-celled or noncellular spherical or
         spiral or rod-shaped organisms lacking chlorophyll that
         reproduce by fission; important as pathogens and for
         biochemical properties; taxonomy is difficult; often
         considered to be plants [syn: bacteria, bacterium]