Open data Handbook defines open data as follows "Open data is data that can be freely used, reused and redistributed by anyone - subject only, at most, to the requirement to attribute and sharealike." On 21st there are events such as hackathons, in various locations, such as Open Data Day London 2015. Some organisations are releasing data e.g. the UK's Natural History Museum "as part of International Open Data Day ... is opening up its digital collections and research data through its new Data Portal."
The Open Data Day website is here http://opendataday.org/
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