A demagogue (/ˈdɛməˌɡɒɡ/; from Ancient Greek δημαγωγός (dēmagōgós) 'popular leader, mob leader'; from Ancient Greek δῆμος (dêmos) 'people, populace' and ἀγωγός (agōgós) 'leading, guiding'), or rabble-rouser, is a political leader in a democracy who gains popularity by arousing the common people against elites, especially through oratory that whips up the passions of crowds, appealing to emotion by scapegoating out-groups, exaggerating dangers to stoke fears, lying for emotional effect, or other rhetoric that tends to drown out reasoned deliberation and encourage fanatical popularity. Demagogues overturn established norms of political conduct, or promise or threaten to do so.

Historian Reinhard Luthin defined demagogue as "a politician skilled in oratory, flattery and invective; evasive in discussing vital issues; promising everything to everybody; appealing to the passions rather than the reason of the public; and arousing racial, religious, and class prejudices—a man whose lust for power without recourse to principle leads him to seek to become a master of the masses. He has for centuries practiced his profession of 'man of the people'. He is a product of a political tradition nearly as old as western civilization itself."

Demagogues have appeared in democracies since ancient Athens. Demagogues exploit a fundamental weakness in democracy: Because ultimate power is held by the people, it is possible for the people to give that power to someone who appeals to the lowest common denominator of a large segment of the population. Demagogues have usually advocated immediate, forceful action to address a crisis while accusing moderate and thoughtful opponents of weakness or disloyalty. Many demagogues elected to high executive office have unraveled constitutional limits on executive power and tried to convert their democracy into a dictatorship, sometimes successfully.

A demagogue, in the strict signification of the word, is a 'leader of the rabble'.

— James Fenimore Cooper, "On Demagogues" (1838)

Demagogue, a term originally referring to a leader of the common people, was first coined in ancient Greece with no negative connotation, but eventually came to mean a troublesome kind of leader who occasionally arose in Athenian democracy. Even though democracy gave power to the common people, elections still tended to favor the aristocratic class, which favored deliberation and decorum. Demagogues were a new kind of leader who emerged from the lower classes. Demagogues relentlessly advocated action, usually violent—immediately and without deliberation.^{[citation needed]}

The term "demagogue" has been used to disparage leaders perceived as manipulative, pernicious, or bigoted. However, what distinguishes a demagogue can be defined independently of whether the speaker favors or opposes a certain political leader. A demagogue is defined by how they gain or hold democratic power: by exciting the passions of the lower classes and less-educated people in a democracy toward rash or violent action and breaking established democratic institutions such as the rule of law. James Fenimore Cooper in 1838 identified four fundamental characteristics of demagogues:

Their methods are known as demagoguery or demagogy. The central feature of demagoguery is persuasion by means of passion, shutting down reasoned deliberation and consideration of alternatives. While many politicians in a democracy make occasional small sacrifices of truth, subtlety, or long-term concerns to maintain popular support, demagogues do these things relentlessly and without self-restraint. Demagogues "pander to passion, prejudice, bigotry, and ignorance, rather than reason."