How to … Understand synonyms
Synonyms are names that are not, under current understanding, the correct names for recognized taxa.
Synonyms can arise because of imperfect communication, or because of inadequate knowledge. Synonyms add to the burden of taxonomy – it would be great if there were no synonyms and every recognized taxon had only one name, but this is not the case, mostly for historical reasons. You should always aim to avoid creating a (future) synonym by doing careful work and being taxonomically and nomenclaturally very careful.
There are two main ways by which synonyms are generated, and these give rise to two classes of synonyms:
Two or more names may be published based on the same type. This could be a nomenclaturally superfluous new name and the name that should have been used instead. This happened relatively frequently in the past, when communication between taxonomists was often limited. It should never happen now. It could also be a new combination, name at new rank, or replacement name, which automatically has the same type as its basionym or replaced synonym. Synonyms based on the same type are termed homotypic synonyms (sometimes also termed nomenclatural synonyms).
Two or more named taxa, based on different types, are now regarded as belonging to a single taxon. This still happens, and will continue to happen, through careful taxonomic work demonstrating that taxa that were once believed to be different and separate are in fact the same. Synonyms based on different types are termed heterotypic synonyms (sometimes also termed taxonomic synonyms).
Different journals have different styles for laying out synonymy in taxonomic treatments, but in general all nomenclatural synonyms are bundled together in one paragraph, followed by information about their type, whereas taxonomic synonyms (which may themselves be bundles of nomenclatural synonyms) are placed in separate paragraphs.
For more information, see Chapter 2 Basic concepts and terms in The Code Decoded.