Magnitude
A number describing the energy an earthquake released at its source, on a logarithmic scale.
Magnitude is the single number that describes how much energy an earthquake released where it started. It is logarithmic, so each whole step up means about 32 times more energy. A magnitude 6 releases roughly 1,000 times the energy of a magnitude 4.
Modern magnitudes are usually moment magnitude (Mw), based on the size of the fault rupture, rather than the older Richter scale that saturates for large events. Magnitude is a property of the earthquake itself and does not change with distance, unlike intensity, which describes shaking at a specific place.