X21 may refer to:
X.21 is an interface specification for differential communications introduced in the mid-1970s by the ITU-T. X.21 was first introduced as a means to provide a digital signaling interface for telecommunications between carriers and customers' equipment. This includes specifications for DTE/DCE physical interface elements, alignment of call control characters and error checking, elements of the call control phase for circuit switching services, and test loops.
Kantianism is the philosophy of Immanuel Kant, a German philosopher born in Königsberg, Prussia. The term Kantianism or Kantian is sometimes also used to describe contemporary positions in philosophy of mind, epistemology, and ethics.
The Boeing Bird of Prey is an American black project aircraft, intended to demonstrate stealth technology. It was developed by McDonnell Douglas and Boeing in the 1990s. The company provided $67 million of funding for the project; it was a low-cost program compared to many other programs of similar scale. It developed technology and materials which would later be used on Boeing's X-45 unmanned combat air vehicle. As an internal project, this aircraft was not given an X-plane designation. There are no public plans to make this a production aircraft. It is characterized as a technology demonstrator.

Peter Reyner Banham was an English architectural critic and writer best known for his theoretical treatise Theory and Design in the First Machine Age (1960) and for his 1971 book Los Angeles: The Architecture of Four Ecologies. In the latter he categorized the Los Angeles experience into four ecological models and explored the distinct architectural cultures of each. A frequent visitor to the United States from the early 1960s, he relocated there in 1976.

The Hungarian men's national ice hockey team is the national ice hockey team of Hungary. They have participated in the IIHF European Championships, the IIHF World Hockey Championships and the Olympic Games since 1928. A consistent participant of the annual World Championship, Hungary has played at the Olympics three times, most recently in 1964. They are currently ranked 19th in the world by the IIHF.
Banham Conversions was a coachbuilder and manufacturer of kit cars from the late 1970s until 2004. The company, based in Rochester, Kent, was founded by Paul Banham and started off as a coachbuilder, converting vehicles into convertibles. The company built convertible versions of high-end vehicles such as the Ferrari 400, Aston Martin DBS and V8, and the Rolls-Royce Corniche during this time.
The difference-map algorithm is a search algorithm for general constraint satisfaction problems. It is a meta-algorithm in the sense that it is built from more basic algorithms that perform projections onto constraint sets. From a mathematical perspective, the difference-map algorithm is a dynamical system based on a mapping of Euclidean space. Solutions are encoded as fixed points of the mapping.
Banham is an English village and civil parish in the county of Norfolk, about 7 miles (11 km) north of Diss, 12 miles (19 km) east of Thetford and 20 miles (32 km) south-west of Norwich. It is home to Banham Zoo, a private collection open to the public for more than 40 years, which houses over 2000 animals. The Church of England parish church, dedicated to St Mary the Virgin, is a Grade I listed building. The name of the village derives from "Bean homestead/village", or perhaps "hemmed-in land where beans grow".
Arthur Dunn Airpark is a public-use airport located 2 miles (3.2 km) northwest of the central business district of the city of Titusville in Brevard County, Florida, United States. The airport is publicly owned and is administered under the cognizance of the Titusville-Cocoa Airport Authority. The airport has one paved runway that is 2,961 feet in length and a turf runway that is 1,805 feet in length.
Banham may refer to:
The MARC-8 charset is a MARC standard used in MARC-21 library records. The MARC formats are standards for the representation and communication of bibliographic and related information in machine-readable form, and they are frequently used in library database systems. The character encoding now known as MARC-8 was introduced in 1968 as part of the MARC format. Originally based on the Latin alphabet, from 1979 to 1983 the JACKPHY initiative expanded the repertoire to include Japanese, Arabic, Chinese, and Hebrew characters, with the later addition of Cyrillic and Greek scripts. If a character is not representable in MARC-8 of a MARC-21 record, then UTF-8 must be used instead. UTF-8 has support for many more characters than MARC-8, which is rarely used outside library data.
The allied naval bombardments of Japan took place during the last weeks of the Pacific War in 1945, in which warships of the United States Navy, the Royal Navy and the Royal New Zealand Navy bombarded industrial and military facilities in Japan. Most of these bombardments were conducted by battleships and cruisers, and caused heavy damage to several of the targeted factories, as well as nearby civilian areas. A major goal of the attacks was to provoke the Japanese military into committing some of its reserve force of aircraft into battle. However, the Japanese did not attempt to attack the Allied bombardment forces, and none of the involved warships suffered any damage.
HMS Thracian was an S-class destroyer built for the Royal Navy during the First World War.
Xelabus Limited is an independent bus and coach operator, based in Eastleigh, Hampshire. It primarily operates public bus services within the Southampton area. All their operations come under their parent, Xelagroup Ltd.

Ellinair was a Greek airline headquartered in Thessaloniki operating scheduled and charter flights.
"Tell me tell me!!" is a single by Japanese pop idol group Dorothy Little Happy. It is tenth official single by the group and the first song used in an Anime series. It was scheduled to be released on May 20, 2015 in three editions, two limited and one regular. "Tell me tell me!!" is used as the first ending song of the anime series Jewelpet: Magical Change.
X21 was a Japanese girl idol group. It was formed in 2013 of 21 finalists of the 13th Japan Bishōjo Contest and is affiliated to the entertainment company Oscar Promotion.
The Boeing–Saab T-7 Red Hawk, initially known as the Boeing T-X, is an American–Swedish transonic advanced jet trainer produced by Boeing with Saab. In September 2018, the United States Air Force (USAF) selected it for the T-X program to replace the Northrop T-38 Talon as the service's advanced jet trainer.
Aguri Ōnishi is a Japanese actress, voice actress and singer. She is managed by Link Plan Inc, a subsidiary of Pro-Fit.
The Battle of Hong Kong was one of the first battles of the Pacific War in World War II.