An illustration is a decoration, interpretation or visual explanation of a text, concept or process, designed for integration in published media, such as posters, flyers, magazines, books, teaching materials, animations, video games and films.
The origin of the word "illustration" is late Middle English (in the sense 'illumination; spiritual or intellectual enlightenment'): via Old French from Latin illustratio(n- ), from the verb illustrate.
Contemporary illustration uses a wide range of styles and techniques, including drawing, painting, printmaking, collage, montage, digital design, multimedia, 3D modelling. Most illustrators work on a freelance basis.
Depending on the purpose, illustration may be expressive, stylised, realistic or highly technical.
Specialist areas include:
Technical and scientific illustration communicates information of a technical or scientific nature. This may include exploded views, cutaways, fly-throughs, reconstructions, instructional images, component designs, diagrams. The aim is "to generate expressive images that effectively convey certain information via the visual channel to the human observer"
Technical and scientific illustration is generally designed to describe or explain subjects to a nontechnical audience, so must provide "an overall impression of what an object is or does, to enhance the viewer's interest and understanding".
In contemporary illustration practice, 2D and 3D software is often used to create accurate representations that can be updated easily, and reused in a variety of contexts.
In the art world, illustration has at times been considered of less importance than graphic design and fine art.
Today, however, due in part to the growth of graphic novel and video game industries, as well as increased use of illustration in magazines and other publications, illustration is now becoming a valued art form, capable of engaging a global market.
Original illustration art has been known to attract high prices at auction. The US artist Norman Rockwell's painting "Breaking Home Ties" sold in a 2006 Sotheby's auction for USD15.4 million. Many other illustration genres are equally valued, with pinup artists such as Gil Elvgren and Alberto Vargas, for example, also attracting high prices.
The illustrations of medieval codices were known as illuminations, and were individually hand drawn and painted. With the invention of the printing press during the 15th century, books became more widely distributed, often illustrated with woodcuts.
1600s Japan saw the origination of Ukiyo-e, an influential illustration style characterised by expressive line, vivid colour and subtle tones, resulting from the ink-brushed wood block printing technique. Subjects included traditional folk tales, popular figures and every day life. Hokusai's The Great Wave of Kanazawa is a famous image of the time.
During the 16th and 17th centuries in Europe, the main reproduction processes for illustration were engraving and etching. In 18th Century England, a notable illustrator was William Blake (1757-827), who used relief etching. By the early 19th century, the introduction of lithography substantially improved reproduction quality.
In Europe, notable figures of the early 19th Century were John Leech, George Cruikshank, Dickens illustrator Hablot Knight Browne, and, in France, Honoré Daumier. All contributed to both satirical and "serious" publications. At this time, there was a great demand for caricature drawings encapsulating social mores, types and classes.
The British humorous magazine Punch (1841-2002) built on the success of Cruikshank's Comic Almanac (1827-1840) and employed many well-regarded illustrators, including Sir John Tenniel, the Dalziel Brothers, and Georges du Maurier. Although all fine art trained, their reputations were gained primarily as illustrators.
Historically, Punch was most influential in the 1840s and 1850s. The magazine was the first to use the term "cartoon" to describe a humorous illustration and its widespread use led to John Leech being known as the world's first "cartoonist". In common with similar magazines such as the Parisian Le Voleur, Punch realised good illustration sold as well as good text. With publication continuing into the 21st Century, Punch chronicles a gradual shift in popular illustration, from reliance on caricature to sophisticated topical observation.
From the early 1800s newspapers, mass market magazines, and illustrated books had become the dominant consumer media in Europe and the New World. By the 19th Century, improvements in printing technology freed illustrators to experiment with color and rendering techniques. These developments in printing effected all areas of literature from cookbooks, photography and traveling guides, as well as children's books. Also, due to advances in printing, it became more affordable to produce color photographs within books and other materials. 
In America, this led to a "golden age of illustration" from before the 1880s until the early 20th Century. A small group of illustrators became highly successful, with the imagery they created considered a portrait of American aspirations of the time.
Notable names from twentieth century illustrators include Harry N. Abrams who founded the first American company in 1949 which specialized in illustrated book art. His company was known as Abram books which he sold in 1966, but continued to run until 1977.
European publishers of illustrated texts at this time include: M. DuMont Schauberg of Germany who created such works as art books, travel guides, and calendars; Corvina Kiadó, founded in 1954 in Budapest, who published illustrated texts in many languages; Polish publisher Arkady (1957) which was founded in Warsaw and produced books on art and architecture. Other such European cities of the twentieth century who published illustrated texts include Paris, Milan, St. Petersburg, and Cologne. 
Media related to Illustrations at Wikimedia Commons
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