Metadata

3 definitions:

1.
noun
data about data; "a library catalog is metadata because it describes publications"
(Dictionary.com)

2.
Wikipedia
prefers to use the word 'metacontent', :
'Metadata (metacontent) is traditionally found in the card catalogs of libraries. As information has become increasingly digital, metadata is also used to describe digital data using metadata standards specific to a particular discipline. By describing the contents and context of data files, the quality of the original data/files is greatly increased. For example, a webpage may include metadata specifying what language it's written in, what tools were used to create it, and where to go for more on the subject, allowing browsers to automatically improve the experience of users.'
with the word metadata fitting with the following: 'Structural metadata, the design and specification of data structures ... would be "data about the containers of data"'
(Wikipedia)

3.
metadata:
Data about data. Metadata describes how and when and by whom a particular set of data was collected, and how the data is formatted. Metadata is essential for understanding information stored in data warehouses and has become increasingly important in XML-based Web applications.
(Webopedia)

Common across all three: information/data about data.