Open data is a bus timetable published on a municipal website that any person is free to access and use for any purpose. It's the open GIS map files (shapefiles) that many municipalities publish. It's the list of tender awards, or the council minutes shared on the municipality's website.
The Open Definition defines open data as:
“Open means anyone can freely access, use, modify, and share for any purpose (subject, at most, to requirements that preserve provenance and openness).”
Open data is data that is available, accessible, able to be participated universally, able to be re-used and redistributed freely, and open licensed. These central principles and concepts of open data are expanded on here:
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.
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.
Re-use and Redistribution: the data must be provided under terms that permit re-use and redistribution including the intermixing with other datasets.
Open Licensed: Open data requires the data to be open licensed. In South Africa, the constitution provides precedence for this in Section 32 And in the Bill of Rights 23(2). Creative Commons provides an easy guide to licensing any work in a variety of open ways (i.e. non-commercial; share and share alike), and Open Data Commons provides a guide and templates specific to licensing open data. Within South Africa, the City of Cape Town has an open data license, which can be found here.
Open Data forms part of a broader "openness movement" that includes open source, open hardware, open content, open education, open educational resources, open government, open knowledge, open access, open science, and the open web.