fleeting

This word captures the sensation of something slipping through your fingers just as you notice it. It carries a poignant, often wistful quality, suggesting that the beauty or importance of the moment is heightened by its brevity. It is frequently used to describe internal states or sensory perceptions that vanish quickly. In contrast to words like brief or short, which are neutral descriptions of duration, fleeting implies a sense of transience and instability. It is the standard choice for describing things that are impossible to hold onto, such as a smile, a feeling of hope, or a passing thought.