It is the extent to which particles spread on either side of the average. However, it is sometimes possible to infer or even deduce the three dimensional form but extreme care is necessary to do so because the shape in two dimensions will be dependent on the orientation of the section throught the object. The three axes are of the same order of magnitude. An example would be quartz and calcite. One of the axes is considerable longer than the others.
Extreme cases are needle shaped or acicular.
One of the axes is considerably shorter than the others. An intermediate case is tabular. Mica is an example. Like a sphere. Shaped like a lens.