Eterna is a term that is commonly synonymous with the concept of "eternity". However, depending on the context in which the word is used, there are many related terms that could serve as suitable synonyms. For instance, some might use "perpetual" or "everlasting" to describe something that is "eterna" in the sense of being unending or infinite. Others might use "timeless" or "enduring" to describe something that is "eterna" in the sense that it has remained relevant or meaningful for a very long period of time. Ultimately, the most appropriate synonyms for "eterna" will depend on the specific meaning that the word is meant to convey.