Taxonomy
Letter TClassification system that groups and names content by shared criteria so it can be recognized and found.
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Working definition
A taxonomy is a classification system: it decides what belongs together, what each group is called, and which criteria get reused.
In Information Architecture it is not a list of pages. It is the model used to label and group content and features so they can be recognized.
Relation to IA
Taxonomy classifies. A sitemap represents a structure. Navigation lets people move through it.