In a directed acyclic graph, what do you call the nodes with in-degree zero?

The question is in the title. What do you call "root nodes" in directed graphs. Is there a commonly accepted term? "Root" implies there's only one, which is not the case. I've scanned half a dozen glossaries before asking. I'm starting to suspect that no, there's no agreed moniker.

I believe the common term for that is source. While a node with 0 out-degree is called a sink.

StackExchange Computer Science Q#156689, answer score: 15