A taxonomy is a collection of controlled vocabulary terms organized into a hierarchical structure. Each term in a taxonomy is in one or more parent-child relationships to other terms in the taxonomy. There may be different types of parent-child relationships in a taxonomy (e.g., whole-part, genus-species, type-instance), but good practice limits all parent-child relationships to a single parent to be of the same type. Some taxonomies allow poly-hierarchy, which means that a term can have multiple parents. This means that if a term appears in multiple places in a taxonomy, then it is the same term. Specifically, if a term has children in one place in a taxonomy, then it has the same children in every other place where it appears.

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A taxonomy has additional meaning specified via whatever the meaning of the hierarchical link is. In a traditional 'taxonomy' the meaning is generalization/specialization or 'is a kind of', depending on what direction you are going. These days the word 'taxonomy' is used to refer to other kinds of hierarchies with different meanings for the links (e.g., part of, broader topic than, instance of). Sloppy taxonomies will not identify explicitly what the meaning of the link is, and there may be different meanings. If a taxonomy has a variety of very carefully defined meanings for the hierarchical link, then it bears a stronger resemblance to an ontology.