Fleeting, a.
Passing swiftly away; not durable; transient; transitory; as, the fleeting hours or moments.
Syn. -- Evanescent; ephemeral. See Transient.
Shades of meaning
Fernald has no entry of its own for fleeting; he lists it under transient.
Where he writes about it:
That which is fleeting is viewed as in the act of passing swiftly by, and that which is fugitive (L. fugio, flee) as eluding attempts to detain it; that which is evanescent (L. evanesco, from e, out, and vanus, empty, vain) as in the act of vanishing even while we gaze, as the hues of the sunset. — from transient