A monarchy is a form of government in which the office of head of state is usually held until death or abdication and is often hereditary and includes a royal house
Royal House
A royal house or royal dynasty consists of at least one, but usually more monarchs who are related to one another, as well as their non-reigning descendants and spouses. Monarchs of the same realm who are not related to one another are usually deemed to belong to different houses, and each house is...

. In some cases, the monarch is elected. These exceptions make it difficult to define "monarchy" precisely; the most objective and comprehensive (albeit circular) definition would seem to be that a monarchy is a government that calls itself a monarchy.

Royalty is a Government in which the attention of the nation is concentrated on one person doing interesting actions. A Republic is a Government in which that attention is divided between many, who are all doing uninteresting things.

Walter Bagehot|Walter Bagehot, in The English Constitution|The English Constitution (1867)

Americans also seem to believe that the monarchy is a kind of mediaeval hangover, encumbered by premodern notions of decorum; the reality is that the British monarchy, for good or ill, is a modern political institution — perhaps the first modern political institution.

Adam Gopnik|Adam Gopnik, in The New Yorker (29 September 1997)

A monarch's neck should always have a noose around it. It keeps him upright.

Robert A. Heinlein in The Cat Who Walks Through Walls|The Cat Who Walks Through Walls (1985)

The monarchy is a political referee, not a political player, and there is a lot of sense in choosing the referee by a different principle from the players. It lessens the danger that the referee might try to start playing.

Conrad Russell, 5th Earl Russell|Conrad Russell, 5th Earl Russell, as quoted in The Spectator (11 January 1997)