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.