"Major" is somewhat fuzzy, but for now I'm thinking 20 killed OR 100 injured.
Last one per wikipedia would have been in 2017, so per Laplace rule of succession prior should in some sense be 1/(2025 - 2017 + 2) = 10%.
References:
https://www.politico.eu/article/uk-police-arrest-terror-probe-iran-terrorism-investigations/,
https://www.telegraph.co.uk/news/2025/05/04/iranian-terror-attack-uk-hours-away-from-being-launched/,
https://en.wikipedia.org/wiki/Terrorism_in_Europe#Deadliest_attacks
Update 2025-05-06 (PST) (AI summary of creator comment): The creator has clarified that the primary criteria for an event to resolve this market are:
It is a terrorist attack in Europe.
It results in 20 killed OR 100 injured.
An event meeting these criteria will count even if it is not listed on the specific Wikipedia page (Terrorism in Europe#Deadliest_attacks) that was referenced in the original description (in the context of baserate calculation). The 2024 Magdeburg car attack was provided as an example of such an event that would count.
1,000Mmh, <https://en.wikipedia.org/wiki/2024_Magdeburg_car_attack> wasn't in the wikipedia list, but would count, so the baserate should be higher.