Definition of microorganism noun from the Oxford Advanced Learner's Dictionary

 

microorganism

 noun
noun
BrE BrE//ˌmaɪkrəʊˈɔːɡənɪzəm//
 
; NAmE NAmE//ˌmaɪkroʊˈɔːrɡənɪzəm//
 
(specialist)
 
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a very small living thing that you can only see under a microscope