Previous research has established the importance of definitions in mathematics and have distinguished the difference between extracted and stipulated definitions. Much remains unknown, however, about the role definitions serve in developing students' disciplinary literacy. In this study, we analyze how definitions from four US Algebra 1 textbooks and "Khan Academy" define vocabulary in the context of quadratics. Results show that "Khan Academy" tends to use extracted definitions, where textbooks tend to use stipulated definitions. Implications from this study are that there is a need to teach students both stipulated and extracted definitions.