Passing swiftly away; not durable; transient; transitory; as, the fleeting hours or moments
lasting for a markedly brief time; "a fleeting glance"; "fugitive hours"; "rapid momentaneous association of things that meet and pass"; "a momentary glimpse"
Fleeting is used to describe something which lasts only for a very short time. The girls caught only a fleeting glimpse of the driver She wondered for a fleeting moment if he would put his arm around her. = brief + fleetingly fleet·ing·ly A smile passed fleetingly across his face. lasting for only a short time = brief (fleet (16-19 centuries), from fleotan )