There are two definitions for the term "subtree".

The general definition for the term is simply a portion of the [DIT|Definition -- Directory Information Tree], including an entry and all of its subordinates.

The term subtree is also described in [RFC 3672|http://www.ietf.org/rfc/rfc3672.txt] in the form of a subtree specification.  A subtree specification provides a mechanism for grouping entries based on a given set of criteria.