Shane K. Johnson in a good article defining structured, semi-structured, and unstructured data
in terms of where the structure is defined (e.g. externally enforced, self-defined, externally defined):
I tend to think of it as: data, composite or simple, with or without
content. In that context, email is structured composite data (from,
to, subject, date) with unstructured content (message body). The
composite data is structured. The content is unstructured. Though
simple data may or may not be structured. The ‘subject’ data is
unstructured. The ‘to’ data is structured. It is composed of a
local-part (username) and a domain.
While content is unstructured, there may be an implied structure.
I’ve always thought of unstructured data as data whose structure is defined post-creation and depending on specific scenarios. Another characteristic of unstructured data is its structure variability over time.
Original title and link: The Structure of Big Data ( ©myNoSQL)