The Continental European synchronous area is the largest interconnected power grid in the world, with hundreds of millions of customers. Transmission system operators who operate it are members in a Continental Europe regional group in the European Network of Transmission System Operators for Electricity, ENTSO-E . ENTSO-E also includes TSOs operating other related synchronous areas, but they are not covered by this question.
On January 8, 2021, ENTSO-E reported a system split in Continental Area, which was later classified as an extensive incident. It was reported as a close call to a "massive blackout" in Europe by some (Bloomberg).
Previous time there was a blackout of massive scale in Europe was the 2006 European blackout, where 15+ m clients lost power for about 2 hours.
Will there be a very large-scale power outage in continental Europe synchronous grid (> 10 m people affected for more than > 1 hour in several countries) before 2023?
Positive resolution if there is a blackout in more than one country in Continental European Area, affecting total of more than 10 million people for more than 1 hour before 2023-01-01 in relevant timezones.
According to ENTSO-E, Regional Group for Continental synchronous area currently includes Austria, Albania, Belgium, Bosnia-Herzegovina, Bulgaria, Czech Republic, Croatia, Denmark (West), France, Republic of North Macedonia, Germany, Greece, Hungary, Italy, Luxemburg, Montenegro, Nederland, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Switzerland and Turkey (observer member).
Appropriate evidence for resolution criteria having been met include news reports by at least two news agencies such as Reuters, AFP, and AP, preferably supported by later report(s) by ENTSOE https://www.entsoe.eu/news-events/news/. To resolve ambiguities about details, reports from news agencies or national broadcasters from affected countries may also be included.
I did not find any standard definition for massive blackout. Because the context is predisposed to cascading power outages in several countries in the area, this question requires threshold of affecting more than one country, more than 10 million people, and for over 1 hour.
For example, the 2003 Italy blackout is a borderline case but would resolve yes, as a part of Geneva canton in Switzerland was affected. Turkey 2015 would not, as it affected people in only one country.
If there are radical changes in the ENTSO-E Continental Europe area membership that would otherwise render question ambiguous, one can use for resolution any synchronous grid involving more than 50% of the currently participating countries.