There are various synonyms for the word "human lifetime". Some of them include "life span", "existence", "duration of life", "life cycle", "mortal life", "lifespan", "time on earth", and "lifetime duration". Each of these phrases embodies the concept of the time a human being spends living on this planet. Some synonyms are more poetic and emotive than others, such as "life cycle", which implies the cyclical nature of life, or "mortal life", which highlights the finite nature of our existence. Regardless of which synonym is used, the central truth remains: our time on this earth is limited, and it is up to us to make the most of it.