Technical Illustration Definition
An illustration is a visualization or a depiction made by an artist, such as a drawing, sketch, painting, photograph, or other kind of image of things seen, remembered or imagined, using a graphical representation. The word comes from the latin word illustra'tio, illu'stro meaning enlighten, irradiate. Printing is the current process for reproducing illustrations, typically with ink on paper using a printing press. Illustrations can be artistic images illustrating for example a text, poem, fashion, magazines, stamps or a book and very often illustrations were made for children's books. The aim of an illustration is to elucidate or decorate a story, poem or piece of textual information by providing a visual representation of something described in the text. Illustrations can also represent scientific images of flora, medicine or different processes, a biological or chemical processes or technical illustrations to give information on how to use something. Illustrations can be executed in different techniques, like watercolor, gouache, ink, oil, charcoal chalk or woodcut. Paintings are usually original works made on canvas or wood, while illustrations are printed. Illustrations are often carried out as a large-scale industrial process, and is an essential part of publishing and transaction printing.
Technical illustration and Product Illustration is the use of illustration to visually communicate information of a technical nature. Technical illustrations can be component technical drawings or diagrams. Technical illustration in general aim "to generate expressive images that effectively convey certain information via the visual channel to the human observer". Nowadays, many illustration programs are used to create technical illustrations due the need for detailed imaging and repeated updating. Besides the commonplace 2-D Adobe Illustrator, there are many 3-D computer graphics software that are often utilized to create illustration for textbooks, especially scientific ones. Technical illustrations generally describe and explain the subjects to a nontechnical audience. Therefore the visual image should be accurate in terms of dimensions and proportions, and should provide "an overall impression of what an object is or does, to enhance the viewer's interest and understanding".
Paraphrased from Wikipedia: http://en.wikipedia.org/wiki/Illustration