Many fungi undergo a reproductive phase in which more than one genetically distinct nuclei (from 2 separate mating types) is present within the same cytoplasm. In the Ascomycota and Basidiomycota, this phase is termed "dikaryotic", whereas in other fungal phyla the phase is "heterokaryotic." What is the difference here? Is it purely number of nuclei per cell, or does the difference depend on how many genetically distinct nuclei are present (2 or >2)?
In this sense, a heterokaryon is a general term, whereas a dikaryon is a specific term. A dikaryon and a trikaryon (although not often seen in literature) are both heterokaryons.