瞬间 (shùnjiān) is a noun meaning 'a split second' — it emphasizes the brevity of a moment, like a snapshot in time. It's more vivid and shorter than 一时 (a while, temporary) or 此刻 (this very moment). 刹那 (chànà) is a near-synonym but is more literary and Buddhist in origin; 转眼 and 一晃 are verb phrases describing time passing quickly, not a static instant.