Ethos is a Greek word corresponding roughly to Modern English's "ethics."

In rhetoric, "ethos" is one of the principles discussed mainly by Aristotle as a component of all argument. Speakers must establish "ethos." On the one hand, this can mean merely "moral competence," but Aristotle broadens this word to encompass expertise and knowledge. When determining whether a given argument is valid or not, one must question the ethos the speaker has established. Violations of ethos can entail some of the following:

• • The speaker has a direct interest in the outcome of the debate (e.g. a person pleading innocence of a crime);
  • The speaker has a vested interest or ulterior motive in the outcome of the debate;
  • The speaker has no expertise (e.g. a farmer giving a speech on space flight carries less expertise than an astronaut).