A threshold concept is discipline-specific, focuses on understanding of the subject and ... has the ability to transform learners' views of the content.” (Zepke, p. 98) It's not the same as a core concept, although that's a useful place to first put the idea. “A core concept is a conceptual ‘building block' that progresses understanding of the subject; it has to be understood, but it does not necessarily lead to a qualitative different view of the subject matter.” (Meyer and Land, p. 4)