Detonation means nuclear explosion. If a nuclear weapon is launched/dropped/etc but the nuclear weapon does not detonate (due to malfunction, interception, etc), that would not count as detonation. If a conventional explosion occurs but no nuclear explosion (for example, a dirty bomb that distributes radioactive material with a conventional explosion), that doesn’t count.

This is question #8 in the Astral Codex Ten 2023 Prediction Contest. The contest rules and full list of questions are available here. Market will resolve according to Scott Alexander’s judgment, as given through future posts on Astral Codex Ten.