VEI

The USGS has a useful site which explains the different levels of the Volcanic Explosivity Index.

This is a measure of the power of a volcanic eruption.

It is a semi-quantitative eruption magnitude scale.

The VEI scale was proposed in 1982 by volcanologists Chris Newhall and Steve Self. They recognized the need for a way to quickly and easily describe explosive volcanic eruptions and their impacts, but also saw the challenge in determining the measurements that best represent the size of a volcanic eruption and that data for historical eruptions can vary in quantity and quality. Some eruptions might have excellent records of ash volume and plume height, whereas others might not have left a recognizable deposit and be known only from the ambiguous written accounts of a distant observer. To address this variability, Newhall and Self decided that it would be best to utilize numerous different datasets to describe the size of explosive eruptions.

Image source: USGS