An organization of levels. Levels belong to hierarchies (which belong to dimensions). A hierarchy can contain only single level or can contain multiple levels. If it contains multiple levels, the higher levels of the hierarchy are less granular then the lower levels. That is, each member of a higher level contains a larger set of records than does a member of a lower level.
In casual usage, a higher level is called the parent of the lower level. However, it is useful to remember that the hierarchy is a actually a hierarchy among members. Thus it is more accurate to state that a member of the higher level is the parent of one or more members of the lower level. Conversely, any member of a lower level is the child of exactly one member of the higher level.
Hierarchies provide additional features beyond those provided by levels; see Hierarchies and Dimensions and Defining Dimensions, Hierarchies, and Levels in Defining Models for InterSystems Business Intelligence.