A period of existence can be described in many ways using synonyms. Some alternatives might include the term duration, denoting the length of time something has existed. Others could use longevity, emphasizing the endurance or lifespan of the subject. The phrase epoch or era could also signify a distinct and significant time period within a broader existence. Additional synonyms could include lifetime, which speaks to the entire duration of someone's or something's life, or tenure, describing the amount of time someone holds a particular position or in a particular location. Finally, one might use the word span, referring to the amount of time from start to end of a given subject's existence. So, there are many words available to describe a "period of existence".