An ontology is a collection of concepts with names and definitions and clear relationships between all the terms, typically with "is a" and "part of" relationships (e.g., a wheel is a part of a car, and a car is a vehicle, and a vehicle is a thing, although typically much more complex). A crucial component of the ontology is the relationships. A similar collection of concepts where the focus is on the concept names and definitions and not the relationships is typically thought of as controlled vocabulary (CV). Thus far the PSI collections, widely used as proteomics-related ontologies, are part way between the two extremes and are described further below.