Developer Certificate Of Origin
In May 2004, the kernel development community decided to standardize on a requirement to adhere to a Developer Certificate of Origin for contributions to the Linux kernel.
The full text of the DCO is:
Developer's Certificate of Origin 1.0 By making a contribution to this project, I certify that: (a) The contribution was created in whole or in part by me and I have the right to submit it under the open source license indicated in the file; or (b) The contribution is based upon previous work that, to the best of my knowledge, is covered under an appropriate open source license and I have the right under that license to submit that work with modifications, whether created in whole or in part by me, under the same open source license (unless I am permitted to submit under a different license), as indicated in the file; or (c) The contribution was provided directly to me by some other person who certified (a), (b) or (c) and I have not modified it.
Here is the online reference for the official DCO: http://www.osdl.org/newsroom/press_releases/2004/2004_05_24_dco.html
Here is an announcement talking about DCO: http://www.osdl.org/newsroom/press_releases/2004/2004_05_24_beaverton.html