A software license is a set of terms which specify how the user can use, modify or distribute the software.

For a software package to be listed on CLiki, its license must usually be free according to the Debian Free Software Guidelines (or DFSG). CLiki pages about licensing (most of which describe specific licenses) are

There are also substantial lists of licenses elsewhere:

Because the Open Source Definition is derived from the DFSG, code licensed under any OSI-compatible license should be ok...