Noun: A set of data that describes and gives information about other data.
Hmmm… that doesn’t really mean much to most people, so what is it really? Well, one of the ways I like to think about it is by considering a book for sale in a bookshop. The book has a title, an author and a price. They are not part of the book itself but describe something about it. The book may also have a subtitle, but this is not a mandatory piece of information – it is optional metadata.