Licensing Open Data is the act of granting (or not granting) authoritative permission or license to others.
Licensing is essentially granting/denying permission to do something.
Licenses can be defined in a wide-variety of methods; some of the most common are certificates, tags, plates, and identification cards/badges. Note that licenses differ from permits, in that generally a permit is temporary; most permits are issued with an expiration date.
Licensing in regards to Openness (and therefore Open Data):
Licenses & Standards at the Open Source Initiative:
Open Source licenses comply with the Open Source Definition, permitting software to be freely used, modified, and shared.
Open Knowledge Definition 1.0
A full set of conditions which must be met in order for a license to be open is available in the Open Knowledge Definition 1.0.)
OSI Open Source Licenses
Popular OSI Licenses Apache License 2.0
BSD 3-Clause "New" or "Revised" license
BSD 2-Clause "Simplified" or "FreeBSD" license
GNU General Public License (GPL)
GNU Library or "Lesser" General Public License (LGPL)
Mozilla Public License 2.0
Common Development and Distribution License
Eclipse Public License
Complete List of OSI Approved Licenses
Guide to Open Licensing
Guide to Open Data Licensing
List of Conformant Licenses
List of Non-Conformant Licenses