different between derailment vs defailment

English

From French déraillement.

derailment (countable and uncountable, plural derailments)

(rail transport) The action of a locomotive or train leaving the rails along which it runs.
(psychiatry) A pattern of discourse (in speech or writing) that is a sequence of unrelated or only remotely related ideas.
An instance of diverting a conversation or debate from its original topic.

From French défaillement.

defailment (plural defailments)

(obsolete) Failure. [17th c.]
- 1624, John Smith, Generall Historie, in Kupperman 1988, p. 120:
  True it is Argall lost his voyage, but we revictualled him, and sent him for England, with a true relation of the causes of our defailments [...].

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