According to ESRI,
Metadata is a summary document providing content, quality, type, creation, and spatial information about a data set
and according to the US Federal Geographic Data Committee,
A metadata record is a file of information ... which captures the basic characteristics of a data or information resource.
Metadata usability is enhanced by standardizing its format, allowing sharing and "intelligent" utilization of the metadata for searching, sorting, and classifying datasets.
Wikipedia page on metadata.