Background and Aim: Word definition is one of the complicated language skills that require education and linguistic awareness. In this study , comparison was made in word definition ability of children between ages of 4.5 to 7.5 years.

Methods: This study was cross-sectional and descriptive-analytical in nature . Participants included 107 girls and boys who where in age group 1 (54-65 months), age group 2 (66-77 months), and age group 3 (78-90 months). They were selected by multistage sampling method and recruited from nurseries and primary schools in 1, 7, and 17 municipal districts of Tehran . Word definition task was performed on each subject. The reliability was assessed by two independent values and the validity was determined by the content. Kruskal- Wallis and Mann-Whitney U statistical methods were used for analysis.

Results: Mean score of the content in word definition was significantly increased by age (p=0.001). In the second and third age groups there was no significant difference in the content of word definition. The most response that used in all ages was the functional response. With increasing age, error rate (p=0.002) and identical (p=0.003) responses significantly decreased however, percentage use of combination II (p<0.001) responses significantly increased.

Conclusion: By increasing age, quality of definitions in terms of content is improved and definitions change from functional and concrete responses to c ombination II definitions.