Content modeling
Letter CDefining which object types exist, which attributes they have, and how they relate — independent of any given screen.
Updated:
Working definition
Content modeling decides which objects exist (a course, a booking, an article), which attributes they have, and which relationships they keep.
In IA the content model is not a wireframe: it is a map of types and links. A sitemap shows screens; the model shows what the system is made of.
Minimal example
A Booking contains Passengers, corresponds to a Flight, and may include Services. Those relationships exist even if the UI does not yet.