I know positively that this term not dictionary, it is registered but used in certain areas restricted in linguistic analysis, as does the asturiano-riojano philosopher Gustavo Bueno in their analysis of the Aristotelian categories. Cononimo is replaced the Greek prefix syn - by the latino with. Synonymous terms are those that mean the same thing having different signifier. The cononimos would have also different significant but not the same meaning even if similar. The Academy regards this similarity the signifier in the paronyms. For example, pain and anguish does not mean the same thing, but have some semantic similarity. It would be cononimos. Most of our synonyms, which we send to our dictionary are cononimos. I don't know if it would be worth making this distinction.