A wiki is a website that which allows its users to add, modify, or delete its content via a web browser usually using a simplified markup language or a rich-text editor. Wikis are powered by wiki software. Most are created collaboratively. See Wikipedia:Wiki.
By this definition, the FamilySearch Research Wiki is definitely a wiki, as are the following:
WeRelate.org and FamilySearch.org's Family Tree program. In both these programs, additions to the programs have been limited to specific fields and there is no access to the underlying wikitext or a rich text editor, but they retain the ability of users to add, modify, or delete content.
Almost uniformly, when people are first introduced to a wiki program, the fact that anyone registered with the wiki can add, modify or delete information is viewed as the wiki's fatal flaw. The questions revolves around the concern that the wiki will evolve into a pile of junk. Although it is entirely counter-intuitive, wikis retain their integrity for two very simple reasons; human nature and the structure of the wiki itself.
If anyone can enter wrong information, why doesn't that destroy the integrity of the wiki? Because it is human nature to correct errors. You may think so, but there is a significant segment of humankind that will automatically correct an error when they see or hear one. But how do we know that these people who go around correcting errors will get it right either? Experience with wikis says that cumulatively the system will work. Errors will get corrected properly and the whole wiki will become more and more reliable.
But what about my crazy aunt (or whoever) that keeps putting in information that I know is off-the-scale wrong? This is where the structure of the wiki comes into play. It is much easier to erase wrong information than it is to put the information into the program in the first place. For example, FamilySearch Family Tree has a "Restore" function that allows any user to reverse an entry back to its original state. In addition, wikis have a built-in notification system to tell interested users of any changes to the page or individuals they are watching.
So, in FamilySearch Family Tree I enter my "correct" family information and someone comes along and changes it to something that is not correct. If I am watching the individual changed, the programs sends me an email telling me that my watched page has changed. I can then click on the link in the email and see the change and change it back if necessary. I can also contact the person who made the incorrect change and gently educate them with the correct sourced information. What if the person doesn't have an email address in the system? Then that person will not be notified when I change the information back to its original state! Simple. The system itself limits the ability of people to change legitimate information.
Any tendency people might have to make improper changes can also be limited by adding sources to the entries. Most rational people would see that the entry was sourced and read the sources to see if they were believable. Irrational people, if they continue to make unwarranted changes, can also be locked out of the system. So who is the ultimate judge of what is correct? The users themselves.