We define a "digital original series" as a multi-episode series where the most recent season was produced or first made available on a streaming platform. Once we define a title as a digital original, we regard the original streaming platform to be the same in all markets. For example, The Handmaid's Tale is considered a Hulu digital original, in all markets, even if Hulu is not currently available in a given territory.
Where a streaming platform has ordered a new season following a cancellation (e.g. Lucifer from Fox), we regard the series to be a network original until the new season is launched by the streaming platform; at that point we regard the series to be a "digital original".
We therefore include, for example, Black Mirror in our definition of a digital original series. Originally on UK's Channel 4 for the first two seasons, Netflix has since acquired the rights and commissioned seasons 3 and 4.
We include developed, acquired and co-licensed originals
Furthermore, we include in our definition all developed originals (titles that were developed, produced and released by the SVOD service that airs them, e.g. Stranger Things) as well as acquired originals (titles developed and produced by a third-party studio, but where the streaming platform has acquired exclusive rights to air the series, e.g. The Crown).
We think it is also fair to treat co-produced and co-licensed titles such as The End of the F*** World and Frontier as digital originals.
With the proliferation of new streaming platforms, sometimes a different platform is considered the original streaming network. Consider for example what the original network for Star Trek:Discovery should be? In our definition we take this to be CBS All Access, despite Netflix owning the exclusive rights to the series in most non-US markets. Star Trek: Discovery is therefore a digital original and we take CBS All Access to be the original platform, not Netflix, in all markets.
We exclude licensed originals and licensed series
We exclude from our definition any licensed originals such as AMC's Better Call Saul and NBC's The Good Place. And, finally, we also exclude all licensed series such as The Office and Friends from our definition.
Note: For more information please read Matthew Ball's insightful piece "How the Paradox of the Term 'Original Series' Explains the Video Industry (Netflix Misunderstandings, Pt. 4)" on Redef.