From American Heritage Dictionary 4th Ed. (En-En)

n.
A logarithmic scale used to express the total amount of energy released by an earthquake. Although the scale has no upper limit, values are typically between 1 and 9, and each increase of 1 represents a 32-fold increase in released energy.

From Britannica Encyclopaedia 2010

The Richter scale was originally devised to measure the magnitude of local earthquakes in southern California as recorded by a specific kind of seismograph. Current scientific practice has replaced the original Richter scale with other scales, including the body-wave magnitude scale and the moment magnitude scale, which have no restrictions regarding distance and type of seismograph used. Nevertheless, the Richter scale is still commonly cited in news reports of earthquake severity.

On the original Richter scale the smallest earthquakes measurable at that time were assigned values close to zero. Since modern seismographs can detect seismic waves even smaller than those originally chosen for zero magnitude, the Richter scale now measures earthquakes having negative magnitudes. Each increase of one unit on the scale represents a 10-fold increase in the magnitude of an earthquake—in other words, numbers on the Richter scale are proportional to the common (base 10) logarithms of maximum wave amplitudes. In theory the scale has no upper limit, but in practice no earthquake has ever been registered above magnitude 9.