Nuke
Nukes were a beatmap moderation tool in the early years of osu! and the forum modding system. Any beatmaps that were considered to be disregarding the rules (or the creator disregarding the Code of Conduct) could be nuked by either the Beatmap Appreciation Team (BAT) or the Global Moderation Team (GMT).
When a beatmap was nuked, it would have a radioactive symbol 
on its beatmap thread. This would signal other members of the BAT/GMT that it was not to be considered for Ranked until significant changes had been made. If a beatmap made the corresponding changes, the icon would be removed and it could proceed with the ranking process.
Nuked beatmaps and their threads in the Graveyard subforum
Beatmaps could be nuked for a multitude of reasons, but the most common ones were:
- Not following the basics of the Ranking Criteria.
- Containing mistimed sections
- Containing randomly placed hit objects on the grid and/or timeline
- Extremely challenging or unconventional beatmaps (e.g. consisting of a single spinner).
- Not following the Code of Conduct.
Nukes technically still exist on the forums, but with the addition of Modding v2 and vetoes they have effectively been replaced and have no use in the current modding system.