Small multiples are a series of small similar pictures, making a point through repetition. The term was popularized by Edward Tufte

Edward Tufte

Edward Rolf Tufte is an American statistician and professor emeritus of political science, statistics, and computer science at Yale University. He is noted for his writings on information design and as a pioneer in the field of data visualization....

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According to Tufte (Envisioning Information, p. 67):

At the heart of quantitative reasoning is a single question: Compared to what? Small multiple designs, multivariate and data bountiful, answer directly by visually enforcing comparisons of changes, of the differences among objects, of the scope of alternatives. For a wide range of problems in data presentation, small multiples are the best design solution.

Example

In the example, the departmental salary expense is charted by month with a dashed line indicating the average for each department. The scales on each panel are different to emphasize the relative change over time compared to the range. Standardizing the scales could provide insight into comparisons in magnitude between the different departments.