Outliner or Outline Processor

An outliner (or outline processor) is a specialized type of text editor (word processor) used to create and edit outlines, which are text files which have a tree structure, for organization. Textual information is contained in discrete sections called "nodes", which are arranged according to their topic–subtopic (parent–child) relationships, sort of like the members of a family tree. When loaded into an outliner, an outline may be collapsed or expanded to display as few or as many levels as desired.

From Wikipedia