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

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

A mathematical entity consisting of a set of values (the carrier set) and a collection of operations that manipulate them. For example, the Integer abstract data type consists of a carrier set containing the positive and negative whole numbers and 0, and a collection of operations manipulating these values, such as addition, subtraction, multiplication, equality comparison, and order comparison. See also: Abstract algebra

A discipline that deals generally with the extraction of information from acoustic signals in the presence of noise and uncertainty. Acoustic signal processing has expanded from the improvement of music and speech sounds and a tool to search for oil and submarines to include medical instrumentation; techniques for efficient transmission, storage, and presentation of music and speech; and machine speech recognition. Undersea processing has expanded to studying underwater weather and long-term global ocean temperature changes, mammal tracking at long ranges, and monitoring of hot vents. These techniques stem from the rapid advances in computer science, especially the development of large, inexpensive memories and ever-increasing processing speeds.

Signal processing is a discipline that deals with the extraction of information from signals. The devices that perform this task can be physical hardware devices, specialized software codes, or combinations of both. In recent years the complexity of these devices and the scope of their applications have increased dramatically with the rapidly falling costs of hardware and software and the advancement of sensor technologies. This trend has made it possible to pursue sophisticated signal-processing designs at relatively low cost. Some notable applications, in areas ranging from biomedical engineering to wireless communications, include the suppression of interference arising from noisy measurement sensors, the elimination of distortions introduced when signals travel through transmission channels, and the recovery of signals embedded in a multitude of echoes created by multipath effects in mobile communications.

A group of software development methods in which requirements and solutions evolve through collaboration between the developers and the customers. Agile methods have significantly changed the practice of software development since their inception around the turn of the twenty-first century. Traditional software development methods emphasize careful preparation and planning, followed by closely monitored development work culminating in the delivery of a finished product. In contrast, agile methods (see figure) emphasize close collaboration with customers in creating small product increments in a very short time, providing opportunities to change product requirements quickly and easily (hence the term "agile"). This article surveys the rise of agile methods and characterizes them by describing Extreme Programming and Scrum, the two most influential agile methods. See also: Software

Radar equipment carried by commercial and military aircraft. These aircraft use airborne radar systems to assist in weather assessment and navigation. Military systems also provide other specialized capabilities such as targeting of hostile aircraft for air-to-air combat, detection and tracking of moving ground targets, targeting of ground targets for bombing missions, and very accurate terrain measurements for assisting in low-altitude flights. Airborne radars are also used to map and monitor the Earth's surface for environmental and topological study.

A well-defined procedure to solve a problem. As such, an algorithm describes how to solve a problem (Fig. 1). The study of algorithms is a fundamental area of computer science. In writing a computer program to solve a problem, a programmer expresses in a computer language an algorithm that solves the problem, thereby turning the algorithm into a computer program. See also: Computer programming

The process or result of the process whereby the amplitude of a carrier wave is changed in accordance with a modulating wave. This broad definition includes applications using sinusoidal carriers, pulse carriers, or any other form of carrier, the amplitude factor of which changes in accordance with the modulating wave in any unique manner. See also: Modulation

Radio communication employing amplitude modulation (AM) of a radio-frequency (RF) carrier wave as the means of conveying the desired intelligence. In AM, the amplitude of the carrier wave is made to vary in response to the fluctuations of a sound wave, television image, or other information to be conveyed. See also: Amplitude modulation; Radio