Evanescent

Part of speech: adjective (a.)

1. Liable to vanish or pass away like vapor; vanishing; fleeting; as, evanescent joys. So evanescent are the fashions of the world in these particulars. Hawthorne.

2. Vanishing from notice; imperceptible. The difference between right and wrong, is some petty cases, is almost evanescent. Wollaston.