The purpose of the present study is to investigate that word frequency has significant influence on noun definitions in Korean. The experimental group was 80 students from Elementary school, Middle school, High school and University. They rated familiarity and wrote definitions for nouns. Noun definitions were analyzed with semantic categories such as "use/purpose," "description," "association/relation," "partial explanation," "explanation," "error," "partial explanation-attribute," "partial explanation-specific class," "partial explanation-nonspecific class," "explanation-specific class," "explanation-nonspecific class." As a result, they showed familiarity for high-frequency nouns. "EXPL" categories that use class terms or critical attributes were used more frequently in definitions of high-frequency nouns compared with low-frequency nouns. They increased with age and errors decreased with age. Word frequency had a significant influence on noun definitions.