“Long list” (or “longlist”) is list of potential nominees from which a “shortlist” will be selected. It can be both a noun or a verb, e.g., “The novel was longlisted for the Man Booker Prize.”
“Shortlist” came first–as early as the 1920s, according to the Oxford English Dictionary–with “long list” showing up in the 1980s. That’s when it showed up in Britain, that is: the first appearance I’m aware of in the U.S. was last year, when the body administering the National Book Awards instituted a ten-book longlist in each category, subsequently to be trimmed to five.
A measure of the term’s unfamiliarity here is the divergence in rendering it: in this Google search, you can see that Time and The New Yorker use “longlist” while NPR and Vulture use “long list.” The concept and the word will probably catch on, because the more nominees, the more interest can be stoked in a particular prize or award. If and when it does, the one-word form will become standard, as it is in Britain.