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

Ravi, a birdwatcher in Karachi, spotted a transient hummingbird amidst the bustling market. It zipped past stalls selling spicy samosas and colorful bangles, leaving Ravi to chase after it, notebook in hand, before it disappeared into the busy crowd, as fleeting as a dream.

Mehmet sat on the edge of Market Square in Helsinki, watching the fountain's spray dance in the crisp air. As he packed his bags to leave at dawn, a family settled their picnic nearby; he was just another transient, passing through before the city changed again.

“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.”