"Derived Works"
As per the OSI:
"The license must allow modifications and derived works, and must allow them to be distributed under the same terms as the license of the original software.
Rationale
The mere ability to read source isn't enough to support independent peer review and rapid evolutionary selection. For rapid evolution to happen, people need to be able to experiment with and redistribute modifications."
http://www.opensource.org/osd.html
"The license must allow modifications and derived works, and must allow them to be distributed under the same terms as the license of the original software.
Rationale
The mere ability to read source isn't enough to support independent peer review and rapid evolutionary selection. For rapid evolution to happen, people need to be able to experiment with and redistribute modifications."
http://www.opensource.org/osd.html