May Day is a public holiday in some regions, usually celebrated on 1 May or the first Monday of May. It is an ancient festival marking the first day of summer, and a current traditional spring holiday in many European cultures. Dances, singing, and cake are usually part of the festivities.

In 1889, May Day was chosen as the date for International Workers' Day by the socialists and communists of the Second International, as well as anarchists, labor activists, and leftists in general around the world, to commemorate the Haymarket affair in Chicago and the struggle for an eight-hour working day. International Workers' Day is also called "May Day", but it is a different celebration from the traditional May Day.

Transactive memory is a psychological hypothesis first proposed by Daniel Wegner in 1985 as a response to earlier theories of "group mind" such as groupthink. A transactive memory system is a mechanism through which groups collectively encode, store, and retrieve knowledge. Transactive memory was initially studied in couples and families where individuals had close relationships but was later extended to teams, larger groups, and organizations to explain how they develop a "group mind", a memory system that is more complex and potentially more effective than that of any of its individual constituents. A transactive memory system includes memory stored in each individual, the interactions between memory within the individuals, as well as the processes that update this memory. Transactive memory, then, is the shared store of knowledge.

According to Wegner, a transactive memory system consists of the knowledge stored in each individual's memory combined with metamemory containing information regarding the different teammate's domains of expertise. The transactive memory system works similarly to external memory, where other members of the group are the external memory aid. Just as an individual's metamemory allows them to be aware of what information is available for retrieval, so does the transactive memory system provide teammates with information regarding the knowledge they have access to within the group. Group members learn who knowledge experts are and how to access expertise through communicative processes. In this way, a transactive memory system can provide the group members with more and better knowledge than any individual could access on their own.

A fourth-generation programming language (4GL) is any computer programming language that belongs to a class of languages envisioned as an advancement upon third-generation programming languages (3GL). Each of the programming language generations aims to provide a higher level of abstraction of the internal computer hardware details, making the language more programmer-friendly, powerful, and versatile. While the definition of 4GL has changed over time, it can be typified by operating more with large collections of information at once rather than focusing on just bits and bytes. Languages claimed to be 4GL may include support for database management, report generation, mathematical optimization, GUI development, or web development. Some researchers state that 4GLs are a subset of domain-specific languages.

The concept of 4GL was developed from the 1970s through the 1990s, overlapping most of the development of 3GL, with 4GLs identified as "non-procedural" or "program-generating" languages, contrasted with 3GLs being algorithmic or procedural languages. While 3GLs like C, C++, C#, Java, and JavaScript remain popular for a wide variety of uses, 4GLs as originally defined found uses focused on databases, reports, and websites. Some advanced 3GLs like Python, Ruby, and Perl combine some 4GL abilities within a general-purpose 3GL environment, and libraries with 4GL-like features have been developed as add-ons for most popular 3GLs, producing languages that are a mix of 3GL and 4GL, blurring the distinction.

In the 1980s and 1990s, there were efforts to develop fifth-generation programming languages (5GL).

The Manual (How to Have a Number One the Easy Way) is a 1988 book by "The Timelords" (Jimmy Cauty and Bill Drummond), better known as The KLF. It is a step-by-step guide to achieving a No.1 single with no money or musical skills, and a case study of the duo's UK novelty pop No. 1 "Doctorin' the Tardis".

A photon sieve is a device for focusing light using diffraction and interference. It consists of a flat sheet of material full of pinholes that are arranged in a pattern which is similar to the rings in a Fresnel zone plate, but a sieve brings light to much sharper focus than a zone plate. The sieve concept, first developed in 2001, is versatile because the characteristics of the focusing behaviour can be altered to suit the application by manufacturing a sieve containing holes of several different sizes and different arrangement of the pattern of holes.

Photon sieves have applications to photolithography. and are an alternative to lenses or mirrors in telescopes and terahertz lenses and antennas.

When the size of sieves is smaller than one wavelength of operating light, the traditional method mentioned above to describe the diffraction patterns is not valid. The vectorial theory must be used to approximate the diffraction of light from nanosieves. In this theory, the combination of coupled-mode theory and multiple expansion method is used to give an analytical model, which can facilitate the demonstration of traditional devices such as lenses and holograms.

A perpetual bond, also known colloquially as a perpetual or perp, is a bond with no maturity date, therefore allowing it to be treated as equity, not as debt. Issuers pay coupons on perpetual bonds forever, and they do not have to redeem the principal. Perpetual bond cash flows are, therefore, those of a perpetuity.

The history of slavery in the Muslim world began with institutions inherited from pre-Islamic Arabia; and the practice of keeping slaves subsequently developed in radically different ways, depending on social-political factors such as the Arab slave trade. Any non-Muslim could be enslaved. Throughout Islamic history, slaves served in various social and economic roles, from powerful emirs to harshly treated manual laborers. Early on in Muslim history slaves provided plantation labor similar to that in the early-modern Americas, but this practice was abandoned after harsh treatment led to destructive slave revolts, the most notable being the Zanj Rebellion of 869–883. Slaves were widely employed in irrigation, mining, and animal husbandry, but most commonly as soldiers, guards, domestic workers, concubines and sex slaves. Many rulers relied on military slaves (often in huge standing armies) and on slaves in administration - to such a degree that the slaves could sometimes seize power. Among black slaves, there were roughly two females to every one male. Two rough estimates by scholars of the numbers of just one group - black slaves held over twelve centuries in the Muslim world - are 11.5 million and 14 million, while other estimates indicate a number between 12 and 15 million African slaves prior to the 20th century.

Islam encouraged the manumission of Muslim slaves as a way of expiating sins. Many early converts to Islam, such as Bilal, were former slaves. In theory, slavery in Islamic law does not have a racial or color basis, although this has not always been the case in practice. In 1990 the Cairo Declaration on Human Rights in Islam declared that "no one has the right to enslave" another human being. Many slaves were imported from outside the Muslim world.

The Arab slave trade was most active in West Asia, North Africa, and Southeast Africa. The Ottoman slave trade exploited the human resources of eastern and central Europe and the Caucasus; the Barbary Coast slave traders raided the Mediterranean coasts of Europe and as far afield as the British Isles and Iceland. In the early 20th century (post-World War I), authorities gradually outlawed and suppressed slavery in Muslim lands, largely due to pressure exerted by Western nations such as Britain and France. Slavery in the Ottoman Empire was abolished in 1924 when the new Turkish Constitution disbanded the Imperial Harem and made the last concubines and eunuchs free citizens of the newly proclaimed republic. Slavery in Iran was abolished in 1929. Mauritania became the last state to abolish slavery - in 1905, 1981, and again in August 2007. Oman abolished slavery in 1970, and Saudi Arabia and Yemen abolished slavery in 1962 under pressure from Britain. However, slavery claiming the sanction of Islam is documented at present in the predominantly Islamic countries of the Sahel, and is also practiced by ISIS and Boko Haram. It is also practiced in countries like Libya and Mauritania - despite being outlawed.

The ouzo effect (also louche effect and spontaneous emulsification) is a milky (louche) oil-in-water emulsion that is formed when water is added to ouzo and other anise-flavored liqueurs and spirits, such as pastis, rakı, arak, sambuca and absinthe. Such emulsions occur with only minimal mixing and are highly stable.

A squircle is a shape intermediate between a square and a circle. There are at least two definitions of "squircle" in use, the most common of which is based on the superellipse. The word "squircle" is a portmanteau of the words "square" and "circle". Squircles have been applied in design and optics.

The Stalin Note, also known as the March Note, was a document delivered to the representatives of the Western Allies (the United Kingdom, France, and the United States) from the Soviet Union in Germany on 10 March 1952. Soviet general secretary and premier Joseph Stalin put forth a proposal for a German reunification and neutralisation with no conditions on economic policies and with guarantees for "the rights of man and basic freedoms, including freedom of speech, press, religious persuasion, political conviction, and assembly" and free activity of democratic parties and organizations.

James Warburg, a member of the United States Senate Committee on Foreign Relations, testified before the committee on 28 March 1952 and observed that the Soviet proposal might be a bluff, but he thought that it seemed "that our government is afraid to call the bluff for the fear that it may not be a bluff at all" and might lead to "a free, neutral, and demilitarised Germany", which might be "subverted into Soviet orbit". That led to an exchange of notes between the West and the Soviet Union, which eventually ended after the West had insistence for a unified Germany to be free to join the European Defence Community and to be rearmed, demands that Stalin rejected.

West German Chancellor Konrad Adenauer and the Western Allies characterised Stalin's move as an aggressive action that attempted to stall the reintegration of West Germany. However, there was later a debate on whether a chance for reunification had been missed. Six years after the exchange, two West German ministers, Thomas Dehler and Gustav Heinemann, blamed Adenauer for not having explored the chance of reunification.