synonym vs synonymum

synonym

noun
  • A word whose meaning is the same as that of another word. 

  • An alternative (often shorter) name defined for an object in a database. 

  • Any name for a taxon, usually a validly published, formally accepted one, but often also an unpublished name. 

  • A word or phrase with a meaning that is the same as, or very similar to, another word or phrase. 

  • Any of the formal names for a taxon, including the valid name (i.e. the senior synonym). 

synonymum

noun
  • Synonym. 

How often have the words synonym and synonymum occurred in a corpus of books? (source: Google Ngram Viewer )