DuctlessAire 21 SEER 24,000 BTU Wi-Fi Ductless Mini Split Air Conditioner and Heat Pump Variable Speed Inverter – 220V/60Hz

Twenty-one, XXI or 21 may refer to:

• 21 (number)
• One of the years 21 BC, AD 21, 1921, 2021

A conditioner is something that improves the quality of another item.

Conditioner may refer to:

• Conditioner (chemistry)
• Conditioner (farming)
• Air conditioner
• Fabric conditioner
• Hair conditioner
• Leather conditioner
• Power conditioner
• The apparatus that contains most of the resurfacing components on an ice resurfacer

In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, atomic, or molecular particles, or small surface irregularities, as distinct from the macroscopic modes of energy transfer, which are thermodynamic work and transfer of matter. For a closed system (transfer of matter excluded), the heat involved in a process is the difference in internal energy between the final and initial states of a system, after subtracting the work done in the process. For a closed system, this is the formulation of the first law of thermodynamics.

Calorimetry is measurement of quantity of energy transferred as heat by its effect on the states of interacting bodies, for example, by the amount of ice melted or by change in temperature of a body.

In the International System of Units (SI), the unit of measurement for heat, as a form of energy, is the joule (J).

With various other meanings, the word 'heat' is also used in engineering, and it occurs also in ordinary language, but such are not the topic of the present article.

The Mini is a small, two-door, four-seat car produced by the British Motor Corporation (BMC) and its successors British Leyland and the Rover Group, from 1959 until 2000. Minus a brief hiatus, Minis were built for four decades over a single generation, as fastbacks, estates, and convertibles. Initially, the Mini was marketed under the Austin and Morris names, as the Austin Seven and Morris Mini-Minor; the Austin Seven was renamed Austin Mini in 1962 and Mini became a marque in its own right in 1969. Retrospectively, the car is known as the "Classic Mini" to distinguish it from the modern MINI family of vehicles produced since 2000 by BMW, who took ownership of the Mini name following their acquisition of Rover Group.

This distinctive two-door car was designed for BMC by Sir Alec Issigonis. Its space-saving transverse engine and front-wheel drive layout – allowing 80% of the area of the car's floorpan to be used for passengers and luggage – influenced a generation of car makers. The front-wheel-drive, transverse-engine layout were used in many other "supermini" style car designs such as Honda N360 (1967), Nissan Cherry (1970), and Fiat 127 (1971). The layout was also adapted for larger subcompact designs. In 1999, the Mini was voted the second-most influential car of the 20th century, behind the Ford Model T, and ahead of the Citroën DS and Volkswagen Beetle. It is also considered an icon of 1960s British popular culture.

The Mini Mark I had three major UK updates: the Mark II, the Clubman, and the Mark III. Within these was a series of variations, including an estate car, a pick-up, a van, and the Mini Moke, a jeep-like buggy. The performance versions, the Mini Cooper and Cooper "S", were successful as both race and rally cars, winning the Monte Carlo Rally in 1964, 1965, and 1967. The Mini was manufactured in England at the Longbridge plant in Birmingham located next to BMC's headquarters and at the former Morris Motors plant at Cowley, as well as in Australia (Victoria Park/Zetland BMC Australia factory) and later also in Spain (Authi), Belgium, Italy (Innocenti, as the Innocenti Mini), Chile, Malta, Portugal, South Africa, Uruguay, Venezuela, and Yugoslavia (IMV). In 1980, British Leyland launched the Mini's follow-up, the Austin Metro, however the Mini outlasted it and continued to be produced at Longbridge until October 2000.

A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic or pneumatic energy.

Mechanical pumps serve in a wide range of applications such as pumping water from wells, aquarium filtering, pond filtering and aeration, in the car industry for water-cooling and fuel injection, in the energy industry for pumping oil and natural gas or for operating cooling towers and other components of heating, ventilation and air conditioning systems. In the medical industry, pumps are used for biochemical processes in developing and manufacturing medicine, and as artificial replacements for body parts, in particular the artificial heart and penile prosthesis.

When a pump contains two or more pump mechanisms with fluid being directed to flow through them in series, it is called a multi-stage pump. Terms such as two-stage or double-stage may be used to specifically describe the number of stages. A pump that does not fit this description is simply a single-stage pump in contrast.

In biology, many different types of chemical and biomechanical pumps have evolved; biomimicry is sometimes used in developing new types of mechanical pumps.

In kinematics, the speed (commonly referred to as v) of an object is the magnitude of the change of its position over time or the magnitude of the change of its position per unit of time; it is thus a non-negative scalar quantity. The average speed of an object in an interval of time is the distance travelled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as the duration of the time interval approaches zero. Speed is the magnitude of velocity (a vector), which indicates additionally the direction of motion.

Speed has the dimensions of distance divided by time. The SI unit of speed is the metre per second (m/s), but the most common unit of speed in everyday usage is the kilometre per hour (km/h) or, in the US and the UK, miles per hour (mph). For air and marine travel, the knot is commonly used.

The fastest possible speed at which energy or information can travel, according to special relativity, is the speed of light in vacuum c = 299792458 metres per second (approximately 1079000000 km/h or 671000000 mph). Matter cannot quite reach the speed of light, as this would require an infinite amount of energy. In relativity physics, the concept of rapidity replaces the classical idea of speed.