This article focuses on exploring students' understanding of the concept of function concerning three main aspects: secondary students' ability to (1) define the concept of function and present examples of functions, (2) solve tasks which asked them to recognize and interpret the concept of function presented in different forms of representation, and (3) solve function problems. Confirmatory factor analysis verified 4 dimensions comprising the conceptual understanding of functions: definition, recognition, interpretation, and problem solving. Furthermore, the important role of the ability to define the concept on the rest abilities was revealed, leading to important didactic implications.