What distinguishes a scientific 'iron planet' from a terrestrial world with a large core?

Answer

Near-total dominance of iron throughout the entire object

The defining characteristic of an "iron planet" is the near-total dominance of iron throughout the entire object, not just having a large core.

What distinguishes a scientific 'iron planet' from a terrestrial world with a large core?
astronomycompositionplanetssolar systemiron