Placeholder name

Placeholder names are words that can refer to things or people whose names do not exist, are temporarily forgotten, irrelevant, or unknown in the context in which they are being discussed.
Linguistic role.
These placeholders typically function grammatically as nouns and can be used for people (e.g. "John Doe, Jane Doe"), objects (e.g. "widget"), locations ("Main Street"), or places (e.g. "Anytown, USA"). They share a property with pronouns, because their referents must be supplied by context; but, unlike a pronoun, they may be used with no referent—the important part of the communication is not the thing nominally referred to by the placeholder, but the context in which the placeholder occurs.
In their "Dictionary of American Slang" (1960), Stuart Berg Flexner and Harold Wentworth use the term "kadigan" for placeholder words. They define "kadigan" as a synonym for "thingamajig". The term may have originated with Willard R. Espy, though others, such as David Annis, also used it (or cadigans) in their writing. Its etymology is obscure—Flexner and Wentworth related it to the generic word "gin" for engine (as in the "cotton gin"). It may also relate to the Irish surname "Cadigan". 
Hypernyms (words for generic categories; e.g., "flower" for tulips and roses) may also be used in this function of a placeholder, but they are not considered to be kadigans.
Examples.
Placeholder words exist in a highly informal register of the English language. In formal speech and writing, words like "accessory", "paraphernalia", "artifact", "instrument", or "utensil" are preferred; these words serve substantially the same function, but differ in connotation.
Most of these words can be documented in at least the 19th century. Edgar Allan Poe wrote a short story entitled "The Literary Life of Thingum Bob, Esq"., showing that particular form to be in familiar use in the United States in the 1840s. In Gilbert and Sullivan's "The Mikado", W. S. Gilbert makes the Lord High Executioner sing of a "little list" which includes:
... apologetic statesmen of a compromising kind,
Such as: What d'ye call him: Thing'em-bob, and likewise: Never-mind,
and 'St: 'st: 'st: and What's-his-name, and also You-know-who:
The task of filling up the blanks I'd rather leave to you.
Some fields have their own specific placeholder terminology. For example, "widget" in economics, engineering and electronics, or "Blackacre" and "John Doe" or "Jane Doe" in law. "X-ray" was originally a placeholder name for an unexplained phenomenon.
Computing.
Placeholder names are commonly used in computing:
Domain names.
Certain domain names in the format example.tld (such as "example.com", "example.net", and "example.org") are officially reserved as placeholders for the purpose of presentation. Various example reserved IP addresses exist in IPv4 and IPv6, such as in IPv4 documentation and in IPv6 documentation.
Geographical locations.
Placeholders such as "Main Street", "Your County", and "Anytown" are often used in sample mailing addresses. "Ruritania" is commonly used as a placeholder country.
"Something"-stan, where "something" is often profanity, is commonly used as a placeholder for a Middle Eastern or South Asian country or for a politically disliked portion of one's own country. Example – "Carjackastan" 
"Timbuktu", which is also a real city, is often used to mean a place that is far away, in the middle of nowhere, or exotic.
"Podunk" is used in American English for a hypothetical small town regarded as typically dull or insignificant, a place in the U.S. that is unlikely to have been heard of. Another example is "East Cupcake" to refer to a generic small town in the Midwestern United States.
Similarly, "the boondocks" or "the boonies" are used in American English to refer to very rural areas without many inhabitants.
In New Zealand English, "Woop Woops" (or, alternatively, "Wop-wops") is a (generally humorous) name for an out-of-the-way location, usually rural and sparsely populated. The similar Australian English "Woop Woop", (or, less frequently, "Woop Woops") can refer to any remote location, or outback town or district. Another New Zealand English term with a similar use is Waikikamukau ("Why kick a moo-cow"), a generic name for a small rural town.
In British English, "Bongo Bongo Land" (or "Bongo-bongo" Land) is a pejorative term used to refer to Third World countries, particularly in Africa, or to a fictional such country.
Military.
Often used in example names and addresses to indicate to the serviceman where to put his own details.
In the British Army, the fictional Loamshire Regiment is used as a placeholder to provide examples for its procedures such as addressing mail or specimen charges for violations of military law.