We derive an analytical expression of equivalent incoherent scatter spectral bandwidth and discuss its practical and interpretative usefulness at different altitude ranges. In the region above 120 km, where collision can be neglected, the equivalent bandwidth is a simple function of electron and ion temperature, ion mass, and ion composition. In the collision-dominated D region, equivalent bandwidth can be used to conveniently assess the effect of negative ions. As an example to illustrate the usefulness of the equivalent bandwidth, we also present nighttime incoherent scatter E region temperature measurement, which has not been reported previously for middle and low latitudes.