Concept learning has long been recognized as a fundamental form of school-related learning. Scholars in a variety of disciplines have identified concepts as a basic unit of instruction. This study was intended to focus on the previous research related to concept learning levels of concepts attainment, providing concept definition, concept examples, nonexamples, and estimate instance difficulty. This study is presented with the desire that it will bring us closer to a comprehensive understanding on the definition and estimate instance difficulty of concept learning.