Lord is an appellation for a person or deity who has authority, control, or power over others, acting as a master, chief, or ruler. The appellation can also denote certain persons who hold a title of the peerage in the United Kingdom, or are entitled to courtesy titles. The collective "Lords" can refer to a group or body of peers.

According to the Oxford Dictionary of English, the etymology of the word can be traced back to the Old English word hlāford which originated from hlāfweard meaning "loaf-ward" or "bread-keeper", reflecting the Germanic tribal custom of a chieftain providing food for his followers. The appellation "lord" is primarily applied to men, while for women the appellation "lady" is used. This is no longer universal: the Lord of Mann, a title previously held by the Queen of the United Kingdom, and female Lord Mayors are examples of women who are styled as "Lord".

Under the feudal system, "lord" had a wide, loose and varied meaning. An overlord was a person from whom a landholding or a manor was held by a mesne lord or vassal under various forms of feudal land tenure. The modern term "landlord" is a vestigial survival of this function. A liege lord was a person to whom a vassal owed sworn allegiance. Neither of these terms were titular dignities, but rather factual appellations, which described the relationship between two or more persons within the highly stratified feudal social system. For example, a man might be lord of the manor to his own tenants but also a vassal of his own overlord, who in turn was a vassal of the King. Where a knight was a lord of the manor, he was referred to in contemporary documents as "John (Surname), knight, lord of (manor name)". A feudal baron was a true titular dignity, with the right to attend Parliament, but a feudal baron, Lord of the Manor of many manors, was a vassal of the King.

The substantive title of "lord of the manor" came into use in the English medieval system of feudalism after the Norman Conquest of 1066. The title "Lord of the Manor" was a titular feudal dignity which derived its force from the existence and operation of a manorial court or court baron at which he or his steward presided, thus he was the lord of the manorial court which determined the rules and laws which were to govern all the inhabitants and property covered by the jurisdiction of the court. To the tenants of a certain class of manor known in Saxon times as Infangenthef their lord was a man who had the power of exercising capital punishment over them. The term invariably used in contemporary mediaeval documents is simply "lord of X", X being the name of the manor. The term "Lord of the Manor" is a recent usage of historians to distinguish such lords from feudal barons and other powerful persons referred to in ancient documents variously as "Sire" (mediaeval French), "Dominus" (Latin), "Lord" etc.

The Scottish title Laird is a shortened form of 'laverd' which is an old Scottish word deriving from an Anglo-Saxon term meaning 'Lord' and is also derived from the middle English word 'Lard' also meaning 'Lord'. The word is generally used to refer to any owner of a landed estate and has no meaning in heraldic terms and its use is not controlled by the Lord Lyon.

Lord is occasionally used as part of a substantive British noble title in its own right:

In the Peerage of Scotland, the members of the lowest level of the peerage have the substantive title "Lord of Parliament" rather than Baron.

The heir to the throne in Scotland holds the title Lord of the Isles.