In recent years, researchers have become more interested in quadratic equations. This study explores the conceptions high school students have concerning quadratic equations with one unknown, while using concept definition and images as theoretical framework. The data was gathered through semi-structured interviews with 14 eleventh grade high school students. Analysis of the data revealed that none of the students could provide an exact and correct definition of a quadratic equation and their attempted definitions were not consistent with the formal (standard) definition of quadratic equation. Moreover, the findings showed that students' concept image of quadratic equation was quite limited and dominated by ideas concerning factoring. Students' conceptions of quadratic equations also showed that participating students lacked three types of prerequisite knowledge: degree of a polynomial, variable and equals sign. In order to enrich students' concept definition and images, both procedurally and conceptually, lessons from this study need to be used in teaching. The implications of the findings are discussed.