The word lady is a term of respect for a woman among English speakers. It is the equivalent of gentleman. It is also a formal title in the United Kingdom. "Lady" is used before the surname of a woman with a title of nobility or honorary title suo jure, or the wife of a lord, a baronet, and a knight, and also before the first name of the daughter of a Duke, Marquess, or Earl throughout the United Kingdom. Once used to describe only women of a high social class, race, community, and status in Europe; now the term is commonly used to refer to any adult woman among English-speakers globally. Informal use of this word is sometimes euphemistic ("lady of the night" for a prostitute) or, in American slang, condescending (equivalent to "mister").
The word comes from Old English hlǣfdige; the first part of the word is a mutated form of hlāf, "loaf, bread", also seen in the corresponding hlāford, "lord". The second part is usually taken to be from the root dig-, "to knead", seen also in dough; the sense development from bread-kneader, or bread-maker, or bread-shaper, to the ordinary meaning, though not clearly to be traced historically, may be illustrated by that of "lord".
The primary meaning of "mistress of a household" is now mostly obsolete, save for the term landlady and in set phrases such as "the lady of the house." This meaning is retained in the southern states of the USA, and also, in the title First Lady for the wife of an elected official. In many European languages the equivalent term serves as a general form of address equivalent to the English Mrs (French Madame, Spanish Señora, Italian Signora, German Frau, Polish Pani, etc.). In those languages it is correct to address a woman whose name is unknown as Madame, Señora, etc., but in polite English usage "lady" has for centuries only normally been a "term of address" in the plural, which is also the case for "gentleman". The singular vocative use was once common but has become mostly confined to poetry. In some dialects it may still be used to address an unknown woman in a brusque manner, often in an imperative or interrogatory context, analogous to "mister" for an unknown male: e.g., "Hey, lady, you aren't allowed in here!" In this usage, the word "lady" is very seldom capitalized when written. The usual English term for politely addressing a woman is Madam or Ma'am.