Majesty (abbreviated HM for His Majesty or Her Majesty, oral address Your Majesty; from the Latin maiestas, meaning 'greatness') is used as a manner of address by many monarchs, usually kings or queens. Where used, the style outranks the style of (Imperial/Royal) Highness, but is inferior to the style of Imperial Majesty. It has cognates in many other languages, especially of Europe.

Originally, during the Roman Republic, the word maiestas was the legal term for the supreme status and dignity of the state, to be respected above everything else. This was crucially defined by the existence of a specific case, called laesa maiestas (in later French and English law, lèse-majesté), consisting of the violation of this supreme status. Various acts such as celebrating a party on a day of public mourning, contempt of the various rites of the state and disloyalty in word or act were punished as crimes against the majesty of the republic. However, later, under the Empire, it came to mean an offence against the dignity of the Emperor.

The term was first assumed by Charles V, who believed that—following his election as Holy Roman Emperor in 1519—he deserved a style greater than Highness, which preceding emperors and kings had used. Soon, Francis I of France and Henry VIII of England followed his example.

After the fall of the Holy Roman Empire, Majesty was used to describe a monarch of the very highest rank— it was generally applied to God. Variations, such as Catholic Majesty (Spain) or Britannic Majesty (United Kingdom) are often used in diplomatic settings where there otherwise may be ambiguity (see a list).

A person with the title is usually addressed as Your Majesty, and referred to as His/Her Majesty, abbreviated HM; the plural Their Majesties is TM. Emperors (and empresses) use [His/Her/Their/Your] Imperial Majesty, HIM or TIM.

Princely and ducal heads usually use His Highness or some variation thereof (e.g., His Serene Highness). In British practice, heads of princely states in the British Empire were referred to as Highness.

In monarchies that do not follow the European tradition, monarchs may be called Majesty whether or not they formally bear the title of King or Queen, as is the case in certain countries and amongst certain peoples in Africa and Asia.

In Europe, the monarchs of Denmark, Norway, Sweden, the United Kingdom, Spain, the Netherlands and Belgium use the style. By contrast, the heads of state of Liechtenstein and Monaco, being principalities, use the inferior style of Serene Highness. Luxembourg, a Grand Duchy, accords its monarch the style of Royal Highness, as accorded to all other members of the Grand-Ducal Family, due to being descendants of Prince Félix of Bourbon-Parma. In the Holy See, the Pope – while ruling as Sovereign of the Vatican City State – uses the spiritual style of Holiness. Moreover, while Andorra is formally a monarchy, its Co-Princes – the bishop of Urgell (appointed by the Pope) and President of France – use the republican and non-royal style of Excellency. Andorra is the only non-hereditary, elective and appointive monarchy in Europe.