Seigneur or lord is an originally feudal title in France before the Revolution, in New France and British North America until 1854, and in the Channel Islands to this day. The seigneur owned a seigneurie, seigneury, or lordship—a form of title or land tenure—as a fief, with its associated obligations and rights over person and property. In this sense, a seigneur could be an individual—male or female, high or low-born—or a collective entity, typically a religious community such as a monastery, seminary, college, or parish. In the wake of the French Revolution, seigneurialism was repealed in France on 4 August 1789 and in the Province of Canada on 18 December 1854. Since then, the feudal title has only been applicable in the Channel Islands and for sovereign princes by their families.
Seigneur descends from Middle French seigneur, from Old French seignor (oblique form of sire), from Latin seniōrem, the accusative singular of senior ("elder"), the comparative form of senex ("old, elderly"). It is a doublet of the English words senior, sir, sire, seignior, sieur, and monsieur and shares the same provenance as the Italian signore, Portuguese senhor, and Spanish señor, which—like mister—referred to feudal lords before becoming general words of respectful address towards men.
The noble title and land title of a seigneur is a seigneurie or lordship, the rights that the seigneur was entitled to is called seigneuriage, and the jurisdiction exercised over the fief was seigneur justicier. The bearers of these titles, rights, and jurisdiction were generally but not exclusively male. A female seigneur was generally known as a seigneuresse or lady. Similarly, they were generally but not exclusively born into the nobility. In French contexts, commoners were known as roturiers.
In English, seigneur is used in historical scholarship to discuss the French seigneurial system. It is also frequently calqued as "lord", the analogous term in the English feudal system.
The term grand seigneur has survived. Today this usually means an elegant, urbane gentleman. Some even use it in a stricter sense to refer to a man whose manners and way of life reflect his noble ancestry and great wealth. In addition, Le Grand Seigneur had long been the name given by the French to the Ottoman sultan. Notre Seigneur Jésus-Christ is the French equivalent of the English Our Lord Jesus Christ.
The word seignorage is also derived from seigneur.
The title is still used in the Channel Islands, self-governing territories in the English Channel which swear fealty to the British Crown as the successor to the Duke of Normandy. In particular, it refers to the Seigneur of Sark, the hereditary ruler of Sark, a jurisdiction of the Bailiwick of Guernsey. The Seigneur of Saint Ouen and the Seigneur of Samarès are titles in the Bailiwick of Jersey. According to the Feudal Dues Law of 1980 of Guernsey, the style of Dame or Seigneur is legally authorized for use by Seigneurs and Dames of Fiefs of the Crown Dependency of Guernsey. 
Guernsey or the Bailiwick of Guernsey is one of the Channel Islands that is a Crown Dependency. Guernsey still has feudal law and legal fiefs in existence today. Each fief has a Seigneur and/or Dame that owns the fief. The Guernsey fiefs and seigneurs have long existed before baronies and are part of Normandy. While nobility has been outlawed in France and Germany, noble fiefs still exist by law in Guernsey. The owners of the fiefs actually convene each year at the Court of Chief Pleas under the supervision of His Majesty's Government. There are approximately 24 private fiefs in Guernsey that are registered directly with the Crown. Some Fief Seigneurs own more than one Fief or have several Fiefs within their Fief territory.