Open Dictionary
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Setting Levels of Definitions at the Open Dictionary

Think of the educational level as "year in school". For example, level 10 means a typical tenth grader in high school should be able to comprehend the material. Level 16 means a four-year college degree in the subject may be necessary, and so on. Level 25 could be interpreted as requiring a PhD plus 4 years of research experience in the field, and so on.