Structural and functional change of the brain by aging affect linguistic ability and, accordingly, decrease semantic processing ability. Due to the process, there is a difficulty in identifying main contents and an appropriate vocabulary is used in terms of language comprehension. Decrease of language meaning and vocabulary processing ability may affect language defining ability that helps understand target vocabulary and draw related vocabulary. Most of the previous studies on defining ability focus on nouns. However, it is necessary to investigate defining ability in various word classes in Korean as verbs and adjectives are frequently used in Korean, as well as nouns (Choi, 2001). This study attempts to investigate nouns, verbs and adjectives defining ability of normal Korean elderly people aged more than 65 years old (the Ministry of Health & Welfare, 2015) and compare their performance with that of the youth between 15 and 29 years old (the Ministry of Employment and Labor, 2013) to present clinical implications. The participants of this study consist of 45 elderly people and 45 young people who were decided to have normal health condition by a simplified mental health test and an aphasia screening test. With regard to test words, total 200 frequently used words for each word class in Korean are divided into high frequency words and low frequency words by the middle percentage (50%), considering frequency of use (Kang and Kim, 2009). Then, total 36 words are selected; 6 words for each top and low frequency words group for each world class. In a defining word task, definition response scores, response times, definition types and error types are analyzed only for words spoken within 30 seconds. For difference in definition scores and response times between two groups by word classes, a repeated measurement ANOVA and a paired t-test as a post-assessment are conducted. For comparison of definition types and error types between two groups by word classes, an independent sample t-test is performed. According to the findings, it is found that there is a difference in definition response scores, response times, definition types and errors types between two age groups and word classes. First, with regard to definition response scores, the elderly group showed significantly lower performance in nouns, verbs and adjectives than the young group. Second, the elderly group made significantly slower response in common nouns, verbs and adjective than the young group, in terms of definition and response time. Third, with regard to definition types of each word class, the elderly group used significantly higher use of functional definition than the young group in common nouns while the group showed significantly frequent use of relational definition for abstract nouns, verbs and adjectives, compared to the young group. Fourth, the elderly group showed significantly higher repetition of target vocabulary and gestures in common nouns than the young group while there is no significant error types in abstract nouns between the age groups. For verbs, the elderly group had more repetition of target vocabulary and gestures than the young group. For adjectives, only repetition of target vocabulary is significantly higher in the elderly group compared to the young group. According to the findings, it is confirmed that drawing a certain meaning from a word and delivering correct information of a word are difficult when age increases. In addition, it is assumed that definition ability to identify the meaning of a word declines because structural and functional change of the brain due to the normal course of aging have effects on the frontal lobe that controls searching, drawing and executing a word and the temporal lobe that control semantic memory and word repository. This study investigates characteristics of defining a world class (e.g. nouns, verbs and adjectives) by normal Korean elderly people. According to the findings, definition ability can be utilized as a useful tool in evaluation and treatment of the semantic area among communication issues caused by normal aging and basic comparison data for assessment and research of disabled groups by aging.