evanescent \e-veh-NE-sehnt\ (adjective) - Tending to vanish like vapor, transient.
"The popular tune by the one-hit-wonder was so evanescent as to disappear from ones mind mere moments after vanishing from the airwaves."
Latin evanescens, present participle of evanescere "to vanish, disappear" from e(x) "from" + vanescere "vanish" from van-us "empty." Akin to "vanish." The noun is "evanescence" and the verb is evanesce "to vanish quickly into thin air."