Amidst the effort to extend its knowledge base, however, there is still a difficulty in understanding the null curriculum. Thus, the general purpose of this paper is to synthesize the concepts in understanding the null curriculum. Following the research method of the systematic review, relevant materials were searched, gathered, and analyzed. Based on the results, it can be concluded that the null curriculum takes multiple dimensions, occurs at hierarchical levels, and can be identified using different frames of reference. When placed in a broader analytical perspective, the overlaps in the intellectual processes dimension, the inconsistencies of the implemented and experiential levels, and the unknown scope of universal curriculum draw challenges in fully understanding the null curriculum. It is recommended that the ground where the concepts intersect should be consistently used, the boundaries of the null curriculum as regards the different levels should be established, and the scope of the universal curriculum as a frame of reference should be explored in research.