Data is open if it can be freely accessed, used, modified and shared by anyone for any purpose - subject only, at most, to requirements to provide attribution and/or share-alike. Specifically, open data is defined by the Open Definition and requires that the data be (1) Legally open: that is, available under an open (data) license that permits anyone freely to access, reuse and redistribute, and (2)  Technically open: that is, that the data be available for no more than the cost of reproduction and in machine-readable and bulk form.

Source: ODH