An ontology is a way of expressing the relations between data. These relations can be visualized through different tools, but for MIMIC-III the web-based tool WebVOWL was used.
The usage might need futher explanation
The easiest way to make use of webowl is to have an accessable link to a .ttl file(can be done through this wiki as an asset). Otherwise you can either create a new file or open a premade(usually in the file format ".ttl"). This can be done through the "Ontology" button as shown in the picture below.
To begin editing the file, you must choose "modes" and press "editing"
In the editing mode it becomes possible to add or remove relations to each of the nodes in the ontology. In the MIMIC ontology, the relations are created in the format of RDF i.e. in triples based on the chosen default constructors (default elements).