Operational taxonomic units (OTUs)

Operational Taxonomic Units (OTUs) are a practical way to group and identify organisms based on DNA sequence similarity. In microbial ecology and other fields, OTUs are used to classify organisms without needing to assign them to a specific taxonomic rank. The level of sequence similarity used to define an OTU can vary, but it is often set at 97% similarity for bacteria and archaea. OTUs enable researchers to analyze and compare diverse microbial communities without relying on traditional taxonomic classifications.

OTUs are based on similarity, where the similarity between a pair of sequences is calculated as the percentage of locations that match in a pair-wise sequence alignment.
Cahana & Iraqi 2020 Anim Models Exp Med. 3: 229-236

Regardless of the sequencing method used, the final results are presented in Operational Taxonomic Units (OTUs), a sequence that normally identifies an organism at the genus or species level.
Cahana & Iraqi 2020 Anim Models Exp Med. 3: 229-236