notable series of school dictionaries widely used in the United States. Their content is based on the theories of Edward Lee Thorndike (Thorndike, Edward L.), an educational psychologist, and Clarence Lewis Barnhart, lexicographer and editor, both pioneers in producing for school-aged readers dictionaries that were more than simplified versions of adult references.

Decisions regarding entries and their definitions were based on data compiled by Thorndike, who applied his educational-psychology expertise to lexicography. These data, which dealt with the frequency of word use in certain kinds of literature at various grade levels, were combined with Thorndike's first-time use for young readers of illustrative examples in definitions. This approach ultimately led to the separate publication of several dictionaries for particular grade-level groups.

After Thorndike's death (1949), Barnhart continued the preparation and editorship of the series. Although each of the dictionaries has undergone continuous revision, the original lexicographical principles are still intact.