Grouping is the accepted term, but does not denote that the subdivision is into disjoint sets, while partitioning does imply this (-E3, +E9). The partitioning attribute may be composed of several attributes, as well as a single attribute. If this is the case, then partition the relation based on the combination of the attribute values, where each unique combination of attribute values distinguishes a set.
The partitioning attribute is used only in value partitioning.