In
Robert A. Heinlein's
Future History books and stories, an
ephemeral is a short-lived
human being, i.e. not a member of the
Howard Families bloodline. While Howards, bred for lifespan, may live for centuries, ephemerals still average the
seventy years or so of
20th century humans.
Close personal relationships between Howards and ephemerals are somewhat ill-advised, as the long-lived Howard partner invariably outlives the ephemeral, and the result can be depressing for both. This does not, of course, stop Lazarus Long or other Howards from falling in love with ephemerals.