thermal unit: see British thermal unit.

atomic mass unit or amu, in chemistry and physics, unit defined as exactly 1/12 the mass of an atom of carbon-12, the isotope of carbon with six protons and six neutrons in its nucleus. One amu is equal to approximately 1.66 × 10^-24 grams.

astronomical unit (AU), mean distance between the earth and sun; one AU is c.92,960,000 mi (149,604,970 km). The astronomical unit is the principal unit of measurement within the solar system, e.g., Mercury is just over 1/3 AU and Pluto is about 39 AU from the sun.

European Currency Unit: see European Monetary System.

British thermal unit, abbr. Btu, unit for measuring heat quantity in the customary system of English units of measurement, equal to the amount of heat required to raise the temperature of one pound of water at its maximum density [which occurs at a temperature of 39.1 degrees Fahrenheit (°F;) ] by 1°F;. The Btu may also be defined for the temperature difference between 59°F; and 60°F;. One Btu is approximately equivalent to the following: 251.9 calories; 778.26 foot-pounds; 1055 joules; 107.5 kilogram-meters; 0.0002928 kilowatt-hours. A pound (0.454 kilogram) of good coal when burned should yield 14,000 to 15,000 Btu; a pound of gasoline or other fuel oil, approximately 19,000 Btu.

or unit trust or open-end trust

Company that invests the funds of its subscribers in diversified securities and issues units representing shares in those holdings. It differs from an investment trust, which issues shares in the company itself. While investment trusts have a fixed capitalization and a limited number of shares for sale, mutual funds make a continuous offering of new shares at net asset value (plus a sales charge) and redeem their shares on demand at net asset value, determined daily by the market value of the securities they hold.

Learn more about mutual fund with a free trial on Britannica.com.

Group of a prescribed size with a specific combat role within a larger military organization. The chief units in the ancient world were the Greek phalanx and the Roman legion. Modern units originated in the 16th–18th century, when professional armies reemerged in Europe after the end of the Middle Ages. Since then the basic units—company, battalion, brigade, and division—have remained in use. The smallest unit today is the squad, which has 7–14 soldiers and is led by a sergeant. Three or four squads make up a platoon, and two or more platoons make up a company, which has 100–250 soldiers and is commanded by a captain or a major. Two or more companies make up a battalion, and several battalions form a brigade. Two or more brigades, along with various specialized battalions, make up a division, which has 7,000–22,000 troops and is commanded by a major general. Two to seven divisions make up an army corps, commanded by a lieutenant general, which with 50,000–300,000 troops is the largest regular army formation, though in wartime two or more corps may be combined to form a field army (commanded by a general), and field armies in turn may be combined to form an army group.

Learn more about military unit with a free trial on Britannica.com.

in full central processing unit

Principal component of a digital computer, composed of a control unit, an instruction-decoding unit, and an arithmetic-logic unit. The CPU is linked to main memory, peripheral equipment (including input/output devices), and storage units. The control unit integrates computer operations. It selects instructions from the main memory in proper sequence and sends them to the instruction-decoding unit, which interprets them so as to activate functions of the system at appropriate moments. Input data are transferred via the main memory to the arithmetic-logic unit for processing (i.e., addition, subtraction, multiplication, division, and certain logic operations). Larger computers may have two or more CPUs, in which case they are simply called “processors” because each is no longer a “central” unit. Seealso multiprocessing.

Learn more about CPU with a free trial on Britannica.com.

Length of the semimajor axis of Earth's orbit around the Sun, 92,955,808 mi (149,597,870 km), often defined simply as the average distance from Earth to the Sun. Direct measurement through the parallax method cannot be used for accurate determinations, because the Sun's glare blots out the light of the background stars necessary to make the measurement. The most precise values have been obtained by measuring the distance from Earth to other objects orbiting the Sun. This indirect method requires an accurate proportional mathematical model of the solar system; once the distance to one planet or other object is determined, then the distance to the Sun can be calculated.

Learn more about astronomical unit (AU) with a free trial on Britannica.com.