Unfortunately, there is no single, official definition of open- source software (OSS). Rather, there are variations on a licensing theme. In other words, OSS can best be defined and understood by a list of required licensing terms that apply to the use, modification, and distribution of the software.

The most widely recognized licensing terms have three core requirements:

• Mandatory availability of source code
  

• The right to use and redistribute the source code freely by anybody to anybody for any purpose
  

• A requirement to make derivative works available on like terms
  

While the Open Source Initiative, a nonprofit organization, promotes one widely accepted definition of "open source," even the OSI Web site offers a menu of alternatives, with the GPL (Gnu Public License), for example, as one substitute definition and a commonly used form of license. The OSI has developed its own certification program for OSS, and there are now more than 40 types of certified OSS licenses, with many more, in varying degrees, based on the consensus OSS requirements.

Many licenses include some but not all of the above OSS requirements, however, and some software is made available under more than one form of license — for example, an OSS license and a proprietary license. Examples include "freeware," which often allows for unrestricted use of the software with no further restrictions on the user apart from copyright notices, or the Perl "Artistic License" and the Sun licenses governing Java, which include restrictions on modifications and distribution of the software.

"Public domain" software, on the other hand, is freely available with no restrictions on use, modification, or distribution, by declaration of the author(s).

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