Open Knowledge define open data as:
Open data is data that can be freely used, re-used and redistributed by anyone - subject only, at most, to the requirement to attribute and sharealike. The full Open Definition gives precise details as to what this means.
To summarize the most important:
If you’re wondering why it is so important to be clear about what open means and why this definition is used, there’s a simple answer: interoperability.
One of the barriers to open data is the concept that publishing open data will destroy commercial advantage. The truth is that it may disrupt your current business model. In many cases the open data model will simply help accelerate the transition. The barrier to consumers taking control and collating information themselves is being lowered by advances in technology. Identification of the threats to your current business model by open data need to be carried out and mitigated, but also the benefits of being able to utilise increased open data available.
One of the barriers to adoption of open data is that there is a relatively small pool of organisations publishing at the moment.
XCRI-CAP (a schema for publishing course record information) was adopted and put to use by Christopher Gutteridge at the University of Southampton, but they had yet to publish their course materials using it in 2015 as no one was actually consuming it. However, it is the classic catch 22, as without more information available, there isn't a market for people to use it.
A successful example is with TfL's open data. You'd have thought that publishing live bus times would have gone on to prove the rule that 'you wait for one bus, then ten come along at once.' Opening up this data hasn't just allowed TFL to implement better bus stop signage, but also opened up opportunities for app makers to produce apps like 'bus countdown' - saving TfL from having to develop and maintain their own app.
The cherry on the cake in this area though is CityMapper. Not only do they consume the TfL feeds, but also feeds from National Rail so that the end-user travel experience is linked up. If a journey requires a bus and a train - it was always up to the end user to link the two together and calculate it. Opening the data has opened up a new market and CityMapper have stepped right in.