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), the natural number following 20 and preceding 22
• The years 21 BC, AD 21, 1921, 2021

24 may refer to:

• 24 (number), the natural number following 23 and preceding 25
• one of the years 24 BC, AD 24, 1924, 2024

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 the thermal energy transferred between systems due to a temperature difference. In colloquial use, heat sometimes refers to thermal energy itself. Thermal energy is the kinetic energy of vibrating and colliding atoms in a substance.

An example of formal vs. informal usage may be obtained from the right-hand photo, in which the metal bar is "conducting heat" from its hot end to its cold end, but if the metal bar is considered a thermodynamic system, then the energy flowing within the metal bar is called internal energy, not heat. The hot metal bar is also transferring heat to its surroundings, a correct statement for both the strict and loose meanings of heat. Another example of informal usage is the term heat content, used despite the fact that physics defines heat as energy transfer. More accurately, it is thermal energy that is contained in the system or body, as it is stored in the microscopic degrees of freedom of the modes of vibration.

Heat is energy in transfer to or from a thermodynamic system by a mechanism that involves the microscopic atomic modes of motion or the corresponding macroscopic properties. This descriptive characterization excludes the transfers of energy by thermodynamic work or mass transfer. Defined quantitatively, the heat involved in a process is the difference in internal energy between the final and initial states of a system, and subtracting the work done in the process. 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).

The Mini (developed as ADO15) is a small, two-door, four-seat car produced by the British Motor Corporation (BMC) and its successors, from 1959 until 2000. Minus a brief hiatus, original Minis were built for four decades and sold during five, from the last year of the 1950s into the last year of the 20th century, over a single generation, as fastbacks, estates, and convertibles.

The original Mini is considered an icon of 1960s British popular culture. 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. 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. 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.

This distinctive two-door car was designed for BMC by Sir Alec Issigonis. It was manufactured at the Longbridge plant in Birmingham, England located next to BMC's headquarters and at the former Morris Motors plant at Cowley near Oxford, in the Victoria Park/Zetland British Motor Corporation (Australia) factory in Sydney, Australia, and later also in Spain (Authi), Belgium, Italy (Innocenti), Chile, Malta, Portugal, South Africa, Uruguay, Venezuela, and Yugoslavia (IMV).

The Italian version of the Mini was produced by Innocenti in Milan and it was sold under the "Innocenti Mini" marque. Innocenti was also producing Lambretta scooters at that time.

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. In 1966, the first-placed Mini (along with nine other cars) was disqualified after the finish, under a controversial decision that the car's headlights were against the rules.

In August 1959, 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 January 1962 and Mini became a marque in its own right in 1969. In 1980, it once again became the Austin Mini, and in 1988, just "Mini" (although the "Rover" badge was applied on some models exported to Japan).

BMW acquired the Rover Group (formerly British Leyland) in 1994, and sold the greater part of it in 2000, but retained the rights to build cars using the Mini name. Retrospectively, the car is known as the "Classic Mini" to distinguish it from the modern, BMW-influenced MINI family of vehicles produced since 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 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.

Variable may refer to:

• Variable (computer science), a symbolic name associated with a value and whose associated value may be changed
• Variable (mathematics), a symbol that represents a quantity in a mathematical expression, as used in many sciences
• Variable (research), a logical set of attributes
• Variable star, a type of astronomical star
• "The Variable", an episode of the television series Lost