Emergence appears to be the idea that certain phenomena of X do not belong to the elements of X, e.g. All the various atoms that make up water are not wet, and so wetness is said to emerge at some point; amino acids and organic molecules aren’t alive, but at some point you can get a living thing out of them.
It’s unclear if emergence is meant to be taken as a fact or an explanation of a fact. My sense is that it is taken as more or less factual. There appears to be an implicit interpretation of the fact, however, one where the structure out of which things emerge is definite, clear, and substantially real whereas the thing that emerges is a ghostly, ethereal question mark. Life doesn’t vanish into organic molecules, but it also is seen as arising entirely a tergo and lacking the clear substantiality of organic molecules.
A hylemorphic account of emergence might go something like this: emergence is a mode of understanding things in the order of material causality, so far as the parts of thing are always in this order. Seen in this line, the ghostly nature of the emergent thing is a symbol for formal causality. It is, if you like, the moment where we recognize that we have crossed over a threshold into an order of causality that is no longer homogeneous with the constituent parts. It’s not that the molecules are more real, they are just more primary in the material order. And so we might make a slight correction to the way we visualize emergence and see it in a hylemorphic way: the emergent reality appears to lack the substantiality of the parts not because it is less real, but because we have recognized a sort of causality that is no longer homogeneous with the causality of the parts.