1. Wild animals are increasingly exposed to human-induced threats in the ocean realm. Cetacean species, as predators and biomonitoring models, are subjected to a variety of stressors that may result in poor health, injuries, and persistent marks. These malformations can easily be documented by photography or video and can be used to infer about the animals' health, especially if combined with long-term photographic identification. However, the value of such information for monitoring is vastly increased if a standardised language is used. We provide a broad definition of deformity, as a general concept for conditions or abnormal features in cetacean species, and categorise externally detected deformities in free-ranging individuals. 2. We define six categories and 58 sub-categories of deformities in cetaceans:anatomical malformations (11 sub-categories), skin lesions (29), anomalous pigmentation (4), injuries due to physical impacts (14), emaciation, and epibionts. Categorisation was based on peer-reviewed literature published between 2000and 2021 (n = 253 studies), comprising 80 of the 101 extant species, documented in studies conducted across 50 countries; reviewed literature included papers from 86 scientific journals, with three journals contributing 100 out of the 253 documents. 4. Overall, and for both Odontoceti and Mysticeti, physical impact was the most reported category; injuries due to physical impacts were documented in 74% of the 253 studies we reviewed. This may be related to the common exposure of cetaceans to intense human marine traffic and fishing activities. 5. Especially with the growing use of open science, a consistent and common language is fundamental for data comparison and to support cetacean research, management and conservation efficiently. We suggest that researchers adopt these definitions and categories when describing abnormalities observed in free-living cetaceans.