transient

one who stays for only a short time

Passing before the sight or perception, or, as it were, moving over or across a space or scene viewed, and then disappearing; hence, of short duration; not permanent; not lasting or durable; not stationary; passing; fleeting; brief; transitory; as, transient pleasure.

from Latin 'transiens', present participle of 'transire' - to go across

“However, he dismissed the contemplation of credit and its transient charms. "I won't dine at all," he said.”

— George Meredith, Complete Project Gutenberg Works of George Meredith

“The isotope 228Ac is a transient member of the thorium series decay chain, which begins with the parent isotope ²³²Th and ends with the stable lead isotope ²⁰⁸Pb.”