1) Cognitive content 2) Content 3) Knowledge about knowledge 4) Mental object 
Metaknowledge or meta-knowledge is knowledge about a preselected knowledge. For the reason of different definitions of knowledge in the subject matter literature, meta-information is or is not included in meta-knowledge. Detailed cognitive, systemic and epistemic study of human knowledge requires a distinguishing of these concepts. but in the comm...
[n] - knowledge about knowledge
noun knowledge about knowledge