To be ontologically parasitic, a thing must exist only in reference to another thing.
For example, in the excellent video "How Many Holes Does a Human Have?", holes are identified as ontologically parasitic. They cannot exist without a thing to reside in. E.g., the holes in a block of Swiss cheese can only exist if that block of Swiss cheese exists. The Stanford Encyclopedia of Philosphy has a pretty interesting article on this as well.
The Stanford encyclopedia also identifies boundaries as being ontologically parasitic; they of course, cannot exist without two objects to be separated by the boundary itself.
I find this topic fascinating, but I'm struggling to think of more examples of this phenomenon. Can you think of any more examples? Are there any papers or books discussing this concept more in-depth?