A physically motivated, theoretical basis for defining scrape-off layer (SOL) widths is proposed. Applying it to theoretical models proposed for transport in the SOL shows how the width is affected by the form of the cross-field transport and the parallel transport to target plates. For a cross-field diffusivity and a parallel loss time , the estimate , with C typically in the range 0.6-1.0 and and evaluated at the SOL-core interface, is found to provide a physically meaningful and accurate expression for the power SOL width .