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:
Availability and Access: the data must be available as a whole and at no more than a reasonable reproduction cost, preferably by downloading over the internet. The data must also be available in a convenient and modifiable form.
Re-use and Redistribution: the data must be provided under terms that permit re-use and redistribution including the intermixing with other datasets.
Universal Participation: everyone must be able to use, re-use and redistribute - there should be no discrimination against fields of endeavour or against persons or groups. For example, ‘non-commercial’ restrictions that would prevent ‘commercial’ use, or restrictions of use for certain purposes (e.g. only in education), are not allowed.
Open Data are online, free of cost, accessible data that can be used, reused and distributed provided that the data source is attributed and shared alike
Open Big Data are massive data sets that sometimes is laborious to process.
Open Data Journals: These journals publish peer reviewed articles describing openly accessible datasets for future reuse.
Open Government Data: Online government data, offered free of cost supported with terms that allow reuse and redistribution.