A homotypic synonym is a name that refers to the same type specimen as another name. Homotypic synonyms are declared through a 'nomenclatural act', that is, published in the scientific literature following the formal rules of the International Code of Nomenclature for algae, fungi, and plants (ICN).

For example, Achnanthes minutissima was transferred to the genus Achnanthidium as Achnanthidium minutissimum. These two names are homotypic synonyms of one another. "Homotypic synonym" is equivalent to "nomenclatural synonym" is equivalent to "objective synonym".

A heterotypic synonym is a name that is based on different type specimens. Such synonyms are the opinions of taxonomists rather than formal, nomenclatural rules.

For example, Gomphonema louisiananum was first described by Kalinsky (1984). Later, this same diatom was described as Gomphonema patrickii by Kociolek and Stoermer (1995), referring to a different type specimen. These two names are heterotypic synonyms because it is someone's opinion that these two types refer to different taxa. "Heterotypic synonym" is equivalent to "subjective synonym".

Diatoms.org emphasizes objective synonyms. Expert contributors and reviewers determine the relevant objective synonyms to include on each species page and list them under the heading, SYNONYMS. We consider the relevant homotypic synonyms to be those that analysts might typically encounter.

We also include names that we consider to be subjective synonyms under the heading, REPORTED AS. This heading also includes errors in identification published in various floras.