English Alternative forms OTU Noun operational taxonomic unit (plural operational taxonomic units) A terminal node in phylogenetic analysis; an organism. External nodes: things under comparison; operational taxonomic units See also hypothetical taxonomic unit References D. Graur and W. Li, Fundamentals of Molecular Evolution, Second Edition (2000) T.A. Brown, Genomes, Third Edition (2007)Wikiwand - on Seamless Wikipedia browsing. On steroids.