The ratio of relevant to irrelevant information in a display. The highest possible Signal-to-Noise ratio is desirable in design
"All communication involves the creation, transmission, and reception of information. During each stage of this process, the form of the information—the signal—is degraded, and extraneous information—noise—is added. Degradation reduces the amount of useful information by altering its form. Noise reduces clarity by diluting useful information with useless information. The clarity of information can be understood as the ratio of remaining signal to added noise."
Universal Principles of Design by William Lidwell, Kritina Holden and Jill Butler
"Maximize the data-ink ratio, within reason""Every bit of ink on a graphic requires a reason. And nearly always that reason should be that the ink presents new information."
The Visual Display of Quantitative Information by Edward Tufte, P. 96