[dpbriggs]

It's the number of children each node can have

[adrianmonk]

Maybe they're asking why the letter "d". That is, why "d-ary" instead of "n-ary" or "k-ary".

[barbecue_sauce]

Yes, that was my intent. Why the choice of the letter "d".

[SamReidHughes]

The letter “n” is already used for the heap size, and “k” is typically a number <= n.