Free and Open Source (FOSS)

Many wiki engines are free and open source. Please note that “free” does not refer to the price here 1). Free refers to the freedom a software offers, eg. to modify the sourcecode and to redistribute the modified sourcecode. A software may give you access to the source code but may not allow you to modify it - such software would not be considered FOSS. More information about the philosophy behind this available at gnu.org.

If it is important to you, you may want to check if there is an active development community, whether they are accepting changes to the code. A very old Last release date might indicate that the project has gone stagnant. You might also be interested in what the exact Licence is.

Obviously this refers to the software distribution, not the textual contents of their wiki homepage, or of your wiki installation (you need to think about content licensing after installing a wiki). Wikis that use a copyleft license for their code often use a different, all-permissive license for the content of the wiki that gets copied into your wiki.