derailment

1. The action of a locomotive or train leaving the rails along which it runs. (countable, uncountable)
2. An instance of diverting a conversation or debate from its original topic. (countable, uncountable)
3. An instance of thwarting or frustrating something. (countable, uncountable)
4. A pattern of discourse (in speech or writing) that is a sequence of unrelated or only remotely related ideas. (countable, uncountable)

Plural derailments