A passing fancy is a temporary interest or enthusiasm that quickly fades away. It can be described as a FAD, whim, PHASE, or flavor of the week. A FAD is a short-lived trend, usually involving a particular style of clothing or activity. A whim is an unpredictable or sudden desire or change of mind. A PHASE is a period of time during which something is dominant or active. Finally, a flavor of the week is a person or thing that is popular for a short period of time.