An index for the quantitative expression of short-period geomagnetic activity is being proposed. The index is based on the peak-to-peak range in 2 (or 2.5) min intervals, and is derived from amplitude-linear (fluxgate) instrumentation. It is easily derived, objective, and has high resolution in both time-scale and amplitude; however, it does not distinguish between different frequencies. Such a simple “general-purpose” index is meant to supplement, rather than to replace, the more specialized indices which are currently used for specific applications. Specifications and criteria are outlined, and examples of applications to specific studies (diurnal and stormtime variabilities) are shown.