Glossary:

a | b | c | d | e | f | g | h | i | j | k | l | m | n | o | p | q | r | s | t | u | v | w | x | y | z

assigned node: A node that is assigned to one or more categories. This can also be called 'categorizing a node', 'classifying a node', and 'tagging a node'; all of these expressions are equivalent to 'assigning categories to a node'. Assigned nodes are generally dynamic pieces of content, such as stories and forum topics; categories and containers themselves are not usually assigned categories. Assigned nodes are not strictly part of the category hierarchy; however, in the case that they are, the assigned category of the lightest weight is considered the node's primary parent.
category: A node type that can be used for creating a static page within a hierarchy, and/or for classifying dynamic content. A category must have an enclosing container, which is often (but not necessarily) also its primary parent. Many of a category's characteristics are determined by the settings applied to its enclosing container. A category is equivalent to, and compatible with, the 'term' entity in the taxonomy module.
container: A node type that can be used for grouping categories into a set, for creating a static page within a hierarchy, and/or for defining a particular vocabulary with which to classify content. A container may have zero, one or many parents, which can be either categories or containers. A range of settings can be applied to a container, many of which affect the behaviour of categories enclosed within it. A container is equivalent to, and compatible with, the 'vocabulary' entity in the taxonomy module.
distant parent: Any parent of a container; or any parent of a category that is neither the category's enclosing container, nor another category enclosed within the same container. Distant parents can be enabled or disabled for containers, and for categories within each container. Distant parent functionality potentially allows any category or container to have any other category or container as its parent; although in most cases, only containers require distant parents. Distant parent functionality is also a crucial difference between the category module and the taxonomy module, as it vastly expands the range of possible tree-like structures that can be created.
hidden container: A container that is, in some circumstances, completely bypassed and ignored when processing the category hierarchy. The aim of a hidden container is that it is completely unknown to end users visiting a site; not only is it not linked to by navigational elements such as TOCs and navlinks, but children and neighbors also 'stretch' to bridge the gap that it leaves in the hierarchy. Hidden containers are always viewable in the administration interface.
hierarchy: The structure or set of structures that can be created by arranging categories and containers in a tree-like fashion. A hierarchy can be as simple as a set of containers, each with a flat set of child categories; or it can be as complex as a multiplicity of categories and containers that are all arranged in a single hierarchy, with each node having multiple distant parents. Where multiple parents are enabled, the hierarchy is ultimately determined by finding the primary parent for each node.
neighbor: When processing a category hierarchy in a strict tree manner, a node's neighbors are its previous and next nodes, according to the logic of a pre-order tree traversal (this is a concept from mathematical tree theory). For example, if a category hierarchy were displayed as an infinitely deep table of contents, then a node's neighbors would be those listed immediately above (previous) and below (next) it.
parent: Categories and containers may have other categories or containers as their parents. Categories and containers lie directly below their primary parent in the category hierarchy. Depending on the way that each container (and the site's global settings) is configured, categories and containers may have zero, one, or many parents. A category's parent is different to its enclosing container, although a category often has its enclosing container as its primary parent.
primary parent: The parent of a category or container with the lightest weight. When a category or container has multiple parents, and the hierarchy needs to be determined in a strict tree manner (i.e. where every node has only one parent), each node has only its primary parent considered, and the rest are ignored.
tree-like: A structure that is similar to a tree, except that each node in the tree may have multiple parents. The category module allows structures to be built in either a flat, tree, or tree-like fashion. Where a tree-like structure needs to be processed in a strict tree manner, only the primary parent of each node is considered.
weight: A whole number value (positive or negative) that is assigned to every category or container. Categories and containers are often sorted according to weight, with those of the lightest weight 'floating' to the top, and those of the heaviest weight 'sinking' to the bottom. Where categories or containers have an equal weight, their order is determined alphabetically by their title.
wrapper: A replacement for a Drupal module, which has all the same functions and signatures (i.e. function arguments and return values) as the original module, but which actually communicates with the category module under the surface. Each function in the wrapper simply calls its equivalent in the category module, performing any necessary conversions or checks along the way. A wrapper module appears to other modules to be exactly the same as the original module, but is actually operating according to the category module's specifications. This provides a compatibility layer between existing Drupal modules that depend on core interfaces, and the category module with its new and unknown interfaces. The main wrapper module provided with the category module is the taxonomy wrapper. When this module is substituted for the real taxonomy module, all taxonomy-dependent modules are able to work with the category module.