A nickname, in some circumstances also known as a sobriquet, or informally a "moniker", is an informal substitute for the proper name of a person, place, or thing. It is distinct from a pseudonym, pen name, stage name, or title, although the concepts can overlap. A nickname may be a descriptive and based on characteristics, or it be a variant form of a proper name. Nicknames may be used for convenience by shortening a name, or they may be used to express affection, playfulness, contempt, or to reflect a particular character trait.

The compound word ekename, meaning "additional name", was attested as early as 1303. This word was derived from the Old English word eac, meaning "also", related to eacian, meaning "to increase". By the 15th century, the misdivision of the syllables of the phrase "an ekename" led to its rephrasing as "a nekename". Though the spelling has changed, the meaning of the word has remained relatively stable ever since.

The term "nickname" is often used as an umbrella term to describe any deviation from a person's legal or official name; a nickname may be used exclusively, instead the person's original name, or it may be used in addition to it, with the person being referred to them interchangeably. Nicknames can be classified into several types, based on their origin or purpose. Nicknames may be based on names, physical characteristics, personality traits, geographical origins, or terms of endearment. Some types of nicknames, like diminutives, or shortened names, may be chosen by their bearers; for example, a woman named "Elizabeth" may choose to go by "Liz" because she finds it faster to write and say. Other types of nicknames tend to be bestowed on people by others. These might still be based on the bearer's legal name, or they might be unrelated. Some of these nicknames are affectionate in nature, like pet names or hypocorism, while others are derogatory and used as a form of name calling. Often, nicknames that are bestowed by other people are based on some type of distinctive characteristic, like "Lefty" for a left-handed person.

English nicknames are generally represented in quotes between the bearer's first and last names (e.g., Dwight David "Ike" Eisenhower and Daniel Lamont "Bubba" Franks). It is also common for the nickname to be identified after a comma following the full real name or later in the body of the text, such as in an obituary (e.g., Frankie Frisch, "The Fordham Flash"). Like English, German uses (German-style) quotation marks between the first and last names (e.g., Andreas Nikolaus „Niki“ Lauda). Other languages may use other conventions; for example, Italian writes the nickname after the full name followed by detto "called" (e.g., Salvatore Schillaci detto Totò), in Spanish the nickname is written in formal contexts at the end in quotes following alias (e.g. Alfonso Tostado, alias «el Abulense»), in Portuguese the nickname is written after the full name followed by vulgo or between parenthesis (e.g. Edson Arantes do Nascimento, vulgo Pelé / Edson Arantes do Nascimento (Pelé)) and Slovenian represents nicknames after a dash or hyphen (e.g., Franc Rozman – Stane). The latter may confuse because it resembles an English convention sometimes used for married and maiden names.

In Viking societies, many people had heiti, viðrnefni, or kenningarnöfn (Old Norse terms for nicknames) which were used in addition to, or instead of, the first name. In some circumstances, the giving of a nickname had a special status in Viking society in that it created a relationship between the name maker and the recipient of the nickname, to the extent that the creation of a nickname also often entailed a formal ceremony and an exchange of gifts known in Old Norse as nafnfestr ('fastening a name'). Nicknames are widely attested in Anglo-Saxon England, and similar social models have been applied to these names.

In Bengali society, for example, people will often have two names: a daknam (pet name) which is the name used by family and friends and a bhalonam which is their formal name.

In the United Kingdom, some surnames have nicknames traditionally attached. A man with the surname 'Clark' will be nicknamed 'Nobby'; the surname 'Miller' will have the nickname 'Dusty' (alluding to the flour dust of a miller at work); the surname 'Adams' has the nickname 'Nabby'. Several other nicknames are linked traditionally with surnames, including Chalky White, Bunny Warren, Tug Wilson, and Spud Baker. Other British nicknames allude to a person's origins. A Scotsman may be nicknamed 'Jock', an Irishman 'Paddy' (alluding to Saint Patrick, the patron saint of Ireland), or 'Mick' (alluding^{[clarification needed]} to the preponderance of Roman Catholicism in Ireland), and a Welshman may be nicknamed 'Taffy' (from Welsh Dafydd, David). Some nicknames referred ironically to a person's physical characteristics, such as 'Lofty' for a short person, 'Curly' for a bald man, or 'Bluey' for a redhead.

In Chinese culture, nicknames are frequently used within a community among relatives, friends, and neighbors. A typical southern Chinese nickname often begins with a "阿" followed by another character, usually the last character of the person's given name. For example, Taiwanese politician Chen Shui-bian (陳水扁) is sometimes referred as "阿扁" (A-Bian). In many Chinese communities of Southeast Asia, nicknames may also connote one's occupation or status. For example, the landlord might be known simply as Towkay (simplified Chinese: 头家; traditional Chinese: 頭家; Pe̍h-ōe-jī: thâu-ke), Hokkien for "boss") to his tenants or workers, while a bread seller would be called "Mianbao Shu" 面包叔 (literally, Uncle Bread).