On page 30 of UC, McCloud says 'Defining the cartoon would take up as much space as defining comics...' In place of a definition, he creates a diagram with a photo-like halftone drawing on the left, a series of transitional steps, and a simplified cartoon image on the right. He uses this step diagram as a motif throughout the book. Obviously this diagram doesn't cover many of the characteristics of the cartoon image that make cartoons so powerful. Also, it doesn't represent the actual evolution of cartoon images as practiced by real artists. Artists don't begin with a realistic rendering of a human face; they begin with a simple drawing, and evolve with practice and learning in a number of different directions:
1) Towards ECONOMY OF LINE, in which the maximum effect is produced by every line, and unnecessary lines are eliminated. This is distinct from the naive simple drawing.
2) Towards CARACATURE and other kinds of exaggeration.
3) Towards ANTHROPOMORPHISM.
4) Towards STYLIZATION in rendering.
5) Towards NATURALISM, that is, the copying of nature and the real world.
6) Towards IDEALIZATION, the creation of characters who are more healthy, more beautiful, and more flawless than real persons. Related to idealization is STEREOTYPING, the creation of ideals within a social context-- the ideal cab driver, the perfect mom, the typical mugger.
7) Towards DIFFERENTIATION. This refers to portraying different aspects of the subject. There are four general categories of differentiation: A) Portraying the subject from different angles-- front, back, above, below, etc. B) Portraying the subject from different distances, from long shots to extreme closeups. C) Portraying the subject under different lighting conditions. D) Portraying the subject with different styles; this is similar to stylization, but refers to using different styles on the same subject.
I would define the cartoon as a drawing style that tends to emphasize elements 1 thru 4 above. A more realistic style will tend to emphasize elements 5 thru 7. Of course, all drawing styles used in comics are combinations of all of these elements-- there's no such thing as a pure cartoon, or a purely realistic drawing.
Comics involve drawing the same subjects over and over, from panel to panel and page to page, with the drawings being consistent enough to be interpreted as a single continuity. This requirement makes the following elements especially important:
* Economy of line makes it easier and less time-consuming to render the same subject repeatedly.
* Caracature and anthropomorphism makes it easier to distinguish one subject from another when several subjects are used in the same continuity. They also provide a shortcut in characterization.
* Stylization adds an element of consistency that helps to pull the continuity together.
* Idealization simplifies the drawing process, once the artist has mastered the drawing of a suitable set of idealized and stereotypical portraits; he or she needs only to reproduce these memorized portraits.
* Differentiation allows the artist to vary the subject from panel to panel so that the repetition doesn't become dull.
Note that naturalism does not have comparable qualities-- actually, naturalism is at odds with the repetitious nature of comics. It tends to lack economy of line; it suppresses stylization; it requires the artist to work from a photo or a model instead of from idealized memory. Naturalism does tend to distinguish one subject from another, but it doesn't do so as effectively as caracature or anthropomorphism.
As a result, naturalism is an awkward approach to comics, and is seldom used. Realistic styles are often a skillful synthesis of caracature, idealization and differentiation, with very little naturalism in the mix. But naturalism is the heart and soul of realism-- the synthetic approach to realism lacks conviction, and more sophisticated adult readers tend to find it off-putting.