Information describing the characteristics of data including, for example, structural metadata describing data structures (e.g., data format, syntax, and semantics) and descriptive metadata describing data contents (e.g., information security labels).
Sources:
CNSSI 4009-2015
NIST SP 800-150 under Metadata

  Data about data. For filesystems, metadata is data that provides information about a file’s contents.
Sources:
NIST SP 800-86 under Metadata

  The information associated with a key that describes its specific characteristics, constraints, acceptable uses, ownership, etc. Sometimes called the key's attributes.
Sources:
NIST SP 800-57 Part 2 Rev.1 under Metadata

  The information associated with a key that describes its specific characteristics, constraints, acceptable uses, ownership, etc.; sometimes called the key's attributes.
Sources:
NIST SP 800-175B Rev. 1 under Metadata

  The information associated with a key that describes its specific characteristics, constraints, acceptable uses, ownership, etc.; sometimes called the key’s attributes.
Sources:
NIST SP 800-57 Part 1 Rev. 5 under Metadata

  Information that describes the characteristics of data, including structural metadata that describes data structures (i.e., data format, syntax, semantics) and descriptive metadata that describes data contents (i.e., security labels).
Sources:
NIST SP 800-53 Rev. 5

  Information describing the characteristics of data. This may include, for example, structural metadata describing data structures (i.e., data format, syntax, semantics) and descriptive metadata describing data contents
Sources:
NIST Privacy Framework Version 1.0 under Metadata