The Collaborative International Dictionary of English v.0.48:

Microbiology \Mi`cro*bi*ol"o*gy\, n. [See Microbe; -logy.]
   The branch of biology studying minute organisms, or microbes,
   such as the bacteria, fungi, viruses, and protozoa. --
   Mi`cro*bi`o*log"ic*al, a. -- Mi`cro*bi*ol"o*gist, n.
   [Webster 1913 Suppl. +PJC] Microcephalic

WordNet (r) 3.0 (2006):

microbiology
    n 1: the branch of biology that studies microorganisms and their
         effects on humans