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An i2b2 instance requires an ontology so that researchers can locate " concepts " of interest and locate patients whose medical records contain these " concepts."

The healthcare concepts contained in the ontology are the building blocks of an i2b2 query. The i2b2 ontology is represented in the upper-left corner of the i2b2 webclient (user interface). See the screenshot below.

These concepts are rendered as a collection of nested folders and generally represent child-parent (or is-a) relationships. Researchers locate these concepts by using a hierarchical tree, or taxonomy. When you log in to use the demo i2b2 site (username "demo", password "demouser" are already filled in), you will see the i2b2 ontology tree on the upper-left side of the user interface. We also refer to that listing as a group of ontologies, where each category or top-level taxonomy is called an ontology in its own right. 

In this screenshot, we would say that i2b2 is displaying an ontology with 13 top-level ontology trees or categories, namely: Clinical Trials, Custom Metadata, Demographics, Diagnoses, etc.

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