License proliferation occurs when a person or group starting a new open source project decide to create a new license because they can't find an existing license that suits them perfectly. The typical reasons for this are that they like some terms in one license (maybe around copyright or patent provisions) and other terms from another license (maybe around derivative works).

An example of this proliferation occurred when commercial open source companies wanted a license that included an attribution clause. As there was no such license available they typically added a clause to the Mozilla Public License and ended up with their own license. To counter this proliferation the OSI approved the CPAL license (http://www.opensource.org/licenses/cpal_1.0).

The number and complexity of FOSS licenses is often listed as one of the barriers that prevents wider adoption of FOSS because it increases the risk and cost of adopting and contributing to FOSS projects. For this reason license proliferation is discouraged and FOSS projects are encouraged to use existing licenses.

License proliferation, in the discussion of free-software licensing, is the increase in distinct license texts.

Since the default of copyright is to grant no permission to do anything with the work, and the license text needs to be carefully worded to grant specific permissions to the recipient, these distinct license texts are usually incompatible: a work licensed under one set of terms, if combined somehow with a work licensed under some other set of terms, will usually not result in a work with the requisite permissions for free software (often, it doesn't even result in a coherent license to do anything with the combined work).

This is bad, because combining works is desirable when working with free software, but only if the result is still free software. If the combined license terms aren't compatible, or don't result in free software, a possible saving of effort is denied: the resulting work is subject to all the terms of all the combined license texts, and the overlap is usually more restrictive, sometimes terminally so.

So a lot of effort goes into combatting the natural tendency of clever hackers to think they can be clever license-text-drafters as well. It's much kinder to the recipients of a work if that work is licensed under terms that are:

• written by, or at least with the primary consultation of, professional copyright lawyers who know the kinds of language traps and corner cases to look for; and
• widely-used, so they have been subject to diverse scrutiny in lots of licensing situations; and
• widely acknowledged to result, in all normal cases, in a work that is free software for all its recipients; and
• no more complex than it needs to be to achieve its aims.