This paper aims at understanding the reasons why variants of the derivatives of HAEMA — Greek for blood — occur in medical vocabulary. Aspects of variation are seen from a terminological standpoint, which may differ from that prevailing in lexicography or in natural language processing (NLP). Not only the forms that are spelt in a similar way are considered as variants of a term, but also its synonyms, i.e. all the forms that have a similar definition. However, since hyponyms of a given term differ by their definitions, i.e. have a separate meaning, they are treated as separate terms: variation becomes part of term formation. The examples in English come from medical dictionaries, a glossary in haematology and MEDLINE. Several types of variation are analysed: graphic, morphological, morphosyntactic, syntactic and lexical, together with their possible meanings, and case studies of the use of some of the variants in context are presented.