Background and Purpose. Given that definitional skills are closely related to literacy and reading comprehension, the purpose of this study was to extend the existing literature in definitional skills by empirically investigating the effect of new parameters that may affect word definitions and definitional types of content and form, such as grammatical categories, word structure, and semantic characteristics. Methods. The sample consisted of 5152 recorded oral definitions produced by 322 individuals (pre-schoolers, school-age children, university students, and adults), who were asked to define 16 words orally. Definitions were transcribed and scored on a six-point scale along a continuum that reflects the developmental path of the definitions. Results. The results indicated a significant interaction between grammatical category and word structure for content and form and also between word structure and semantic characteristics only for content. Furthermore, the grammatical category, word structure, and semantic characteristics were strongly associated with specific definitional types for content and form. Implications. This paper broadens our knowledge on definitional skills and offers new insights into the variables that affect the production of definitions.