The Content Metamodel is one of the two parts of the Architecture Content Framework.
A Metamodel is "a model that describes how and with what the architecture will be described in a structured way" (TOGAF9, ch. 3.52, p. 32)
"The content metamodel provided here defines a formal structure for these terms to ensure consistency within the ADM and also to provide guidance for organizations that wish to implement their architecture within an architecture tool." (TOGAF9, ch. 34.1, p. 367)
The other Open Group standard Archimate can be used as a replacement of the Content Metamodel, which is part of the TOGAF Content Framework.