A process of making physical objects directly from digital models, usually layer-by-layer; also known as additive manufacturing. 3D printing has been hailed as a technology that will ultimately revolutionize many sectors of industry, medicine, art, and other fields. As information technology significantly enhances freedom in the digital world, 3D printing brings this freedom to the physical world by blurring the boundary between information and physical products (Fig. 1). See also: Information technology

A series of intermetallic compounds that have a particular crystal structure and the chemical formula A₃B, where A represents a transition element and B can be either a transition element or a nontransition element. Many A15 compounds (see figure) exhibit the phenomenon of superconductivity at relatively high temperatures in the neighborhood of 20 K (−253°C; −424°F) and in high magnetic fields on the order of several tens of teslas (several hundred kilogauss). High-temperature/high-field superconductivity has a number of important technological applications and is a challenging fundamental research area in condensed-matter physics. See also: High magnetic fields; Solid-state physics; Superconductivity; Temperature

A nocturnal, burrowing, insectivorous, piglike mammal (Orycteropus afer) in the order Tubulidentata. The mammalian order Tubulidentata contains a single family, Orycteropodidae, and a single extant aardvark species, Orycteropus afer (Fig. 1). Aardvarks occur throughout Africa south of the Sahara, wherever suitable habitat exists. In previous classifications, aardvarks and the fossil tubulidentates were assigned to the now-invalid order known as Edentata and were considered closely related to ungulates. However, data from mitochondrial and nuclear genes support a relationship among aardvarks, elephant shrews (family Macroscelididae), paenungulates (hyraxes, sirenians, and proboscideans), and golden moles (family Chrysochloridae). These ecologically divergent adaptive types probably originated in Africa; the molecular evidence implies that they may have arisen there from a common ancestor that existed in the Cretaceous Period, when Africa was isolated from other continents. The members of the order Tubulidentata show the results of an extreme adaptation for burrowing and for feeding on small food items (mainly termites). See also: Adaptation (biology); Africa; African mammals; Edentata; Mammalia; Tubulidentata

A plant, Musa textilis, of the banana family that is valuable for its hard fiber (also known as Manila hemp). Abaca (Musa textilis) is a flowering plant native to the Philippines and Borneo. Taxonomically, it is assigned to the order Zingiberales. Abaca is one of the strongest of the hard fibers. Commercially, it is known as Manila hemp. The fiber is obtained from the leafstalks of M. textilis, which is a member of the banana family. The plant (see illustration) resembles the fruiting banana plant, but it is a bit shorter in stature, bears small inedible fruits, and has leaves that stand more erect than those of the banana. Abaca leaves are also slightly narrower, more pointed, and about 1.5–2 m (4.9–6.6 ft) in length. Relatives of abaca grow wild throughout Southeast Asia, but the plant was domesticated long ago in the southern Philippines. Plants that can yield fiber are grown in a few other parts of the world, chiefly Central America and Ecuador. Most commercial production comes from the Philippines and Ecuador. See also: Banana; Fiber crops; Natural fiber; Zingiberales

An early mechanical calculator. The abacus likely originated in Babylonia sometime before 1000 BCE. Over the subsequent centuries, it became a commonplace device in many regions of the world. The abacus is still occasionally used as a cognitive computation aid in the Far East, having developed into two different styles in China and Japan. Both the Chinese and the Japanese styles consist of a frame with a crossbeam. They may be made from many different materials, such as wood or brass. Rods or wires carrying sliding beads extend vertically through the crossbeam (Fig. 1). The Chinese suan pan has two beads above the beam on each rod and five beads below. Each rod of the Japanese soroban carries one bead above and four below. Similar to the abacus in both construction and use are the counting frames used in elementary schools, and which are also often referred to as abacuses. Braille versions of the abacus are available for use by those without sight. See also: Arithmetic; Calculators; Mathematics

Any of the adenosine triphosphate–binding cassette (ABC) transmembrane proteins that use energy to transport various molecules across extracellular and intracellular membranes (cytoplasmic membranes, endoplasmic reticulum, mitochondria, or peroxisomes). ABC lipid transporters consist of two hydrophobic transmembrane domains and two hydrophilic nucleotide-binding folds. These binding folds contain highly conserved sequence motifs (Walker A and B) and are separated by a linker sequence, also known as the signature (C) motif. ABC transporters are organized as full transporters or half transporters depending on the number of transmembrane domains and nucleotide-binding folds. Half transporters must form homodimers or heterodimers to be functional. See also: Adenosine triphosphate (ATP); Lipid; Lipid metabolism; Protein

A major body division of the vertebrate trunk lying posterior to the thorax (chest) and, in mammals, bounded anteriorly by the diaphragm and extending to the pelvis. The abdomen (see illustration) is a major anatomical region of the body. The diaphragm, found only in mammals, separates the abdominal or peritoneal cavity from the pleural and pericardial cavities of the thorax (chest). In all pulmonate vertebrates (possessing lungs or lunglike organs) other than mammals, the lungs lie in the same cavity with the abdominal viscera (internal organs), and this cavity is known as the pleuroperitoneal cavity. See also: Lung; Mammalia; Regional anatomy; Vertebrata

The apparent change in direction of a celestial source of light caused by an observer's component of motion perpendicular to the impinging rays. In astronomy, aberration manifests, for example, as a star appearing aberrantly from what its true position on the sky would be because of the motion of the Earth. The star in question appears to move in the same direction of motion as an observer on the Earth, an effect which would not occur if Earth were stationary with respect to the star. Because all astronomical bodies are in motion relative to one another, aberration is experienced universally, but is only subjectively relevant, based on the observer. For humankind as observers, aberration has played a historically significant role in astronomy and physics, specifically in the development of theories regarding light, electromagnetism, and relativity. See also: Astronomy; Astrophysics; Electromagnetism; Light; Motion; Physics; Star

A departure of an optical image-forming system from ideal behavior. Ideally, such a system will produce a unique image point corresponding to each object point. In addition, every straight line in the object space will have as its corresponding image a unique straight line. A similar one-to-one correspondence will exist between planes in the two spaces.

A material of extreme hardness that is used to shape other materials by a grinding or abrading action. Abrasive materials may be used either as loose grains, as grinding wheels, or as coatings on cloth or paper. They may be formed into ceramic cutting tools that are used for machining metal in the same way that ordinary machine tools are used. Because of their superior hardness and refractory properties, they have advantages in speed of operation, depth of cut, and smoothness of finish.