An airbrush is a small, air-operated tool that atomizes and sprays various media, most often paint, but also ink, dye, and make-up. Spray painting developed from the airbrush and is considered to employ a type of airbrush.

Up until the mid-2000s, it was widely published that the airbrush was invented in 1893, but following research undertaken in collaboration with New York University's Conservation Department, and personal support from Professor Margaret Holben Ellis, a more detailed history emerged, which required many authorities such as Oxford Art to update their dictionaries and references.

Depending on the definition requiring compressed air or not, the first spray painting device that could be called an airbrush was patented in 1876 (U.S. patent 182,389) by Francis Edgar Stanley of Newton, Massachusetts. This worked akin to a diffuser/atomizer and did not have a continuous air supply. Stanley and his twin brother later invented a process for continuously coating photographic plates (Stanley Dry Plate Company) but are perhaps best known for their Stanley Steamer. No artistic images that used this 'paint distributor/atomiser' exist or are as yet known.

According to the research prepared by Professor Andy Penaluna, the first instrument to use a compressed air supply was named the "paint distributor" was developed by Abner Peeler "for the painting of watercolors and other artistic purposes" and used a hand-operated compressor to supply continuous air. It was rather crude, being based on a number of spare parts in a jeweller's workshop such as old screwdrivers and welding torches. It took four years of further development before a working prototype was developed by Liberty Walkup of Mt. Morris, Illinois. Walkup patented the work under the name of "air-brush", a name his wife Phoebe Walkup came up with. The birth of the name 'Air Brush' can be traced to a stakeholders' meeting of the new Air Brush Manufacturing Co. at 7:00 p.m. on October 6, 1883, when the name was formally born. In modern times the date is used to celebrate airbrush art around the world, using the Twitter hashtag #airbrushartistday. Walkup's wife would later go on to be the founder of the Illinois Art School where airbrushing was taught to students from all over the world. In that same rented 4-story building Rockford Air-Brush would be established under Liberty Walkup. Among others, the Walkups taught airbrush technique to American Impressionist master Wilson Irvine at the Air Brush School in Rockford, Illinois. The first certain 'atomizing' type airbrush was invented by Charles Burdick in 1893 and presented by Thayer and Chandler art materials company at the World Columbian Exposition in Chicago. Burdick founded the Fountain Brush Company in the US, and launched the first series of airbrushes onto the market. However, Burdick initially re-cased the Walkup design into a finger-operated instrument, and as many of his designs echoed those being developed by Walkup, a legal row resulted over the name Air Brush. This device was essentially the same as a modern airbrush, resembling a pen and working in a different manner than Peeler's device. Aerograph, Burdick's original company, still makes and sells airbrushes in England. Thayer and Chandler were acquired by Badger Air-Brush Co. in 2000. Badger Air-Brush continues the Thayer and Chandler tradition of manufacturing quality airbrush guns, tools and compressors out of Franklin Park, Illinois.

An airbrush works by passing a stream of fast moving (compressed) air through a venturi, which creates a local reduction in air pressure (suction) that allows paint to be pulled from an interconnected reservoir at normal atmospheric pressure. The high velocity of the air atomizes the paint into very tiny droplets as it blows past a very fine paint-metering component. The paint is carried onto paper or other surfaces. The operator controls the amount of paint being blown by using a variable trigger which manipulates the position of a very fine tapered needle that is the control element of the paint-metering component. An extremely fine degree of atomization is what allows an artist to create such smooth blending effects using the airbrush.

The technique allows for the blending of two or more colors in a seamless way, with one color slowly becoming another color. Freehand airbrushed images, without the aid of stencils or friskets, have a floating quality, with softly defined edges between colors, and between foreground and background colors. A skilled airbrush artist can produce paintings of photographic realism or can simulate almost any painting medium. Painting at this skill level involves supplementary tools, such as masks and friskets, and very careful planning.

Some airbrushes use pressures as low as 20 psi (1.38 bar) while others use pressures in the region of 30-35 psi (2-2.4 bar). Larger "spray guns" as used for automobile spray-painting need 100 psi (6.8 bar) or more to adequately atomize a thicker paint using less solvent. They are capable of delivering a heavier coating more rapidly over a wide area. Even with small artist airbrushes using acrylic paint, artists must be careful not to breathe in the atomized paint, which floats in the air for minutes and can go deep into the lungs. With commercial spray guns for automobiles, it is vital that the painter have a clean air source to breathe, because automotive paint is far more harmful to the lungs than acrylic. Certain spray guns, called High-Volume Low-Pressure (HVLP) spray guns, are designed to deliver the same high volumes of paint without requiring such high pressures.

Airbrushes are usually classified by three characteristics. The first characteristic is the action performed by the user to trigger the paint flow. The second is the mechanism for feeding the paint into the airbrush. The third is the point at which the paint and air mix.