This paper aims to explore the existence of the essential features of the function concept in public secondary students’ definitions of it across grade-level of the scientific and humanities tracks. A sample of 465 secondary students was chosen from 5 public secondary schools located in Beirut. The findings of the study showed that secondary students in the different grade levels lack a thorough conceptualization of what is a function. For instance, the majority of them failed to give definitions that reflect the essential features of the concept. The relation feature was the most one noticed in students’ definitions. Compartmentalization phenomenon was observed where even the students who succeeded to state the uniqueness property, were not truly aware of it and seemed not to consult this property when presented with non-examples of function.