Wikis take a bit of getting used to – there’s a learning curve for both you and the kids at the start – but they’re actually very easy to use and are a brilliant educational tool.
I use the wiki functionality built into Moodle, but there are other free options available to you. I usually start by creating the first page, with enough wiki words for one word per pair of students. I then assign each pair a key word and they begin writing the entry for that.
After this it becomes a free-for-all as the number of wiki-words that need entries writing increases exponentially, and as a result students no longer need to work in pairs. However collisions will still arise where two students try to edit the same page, but this can result in some great (by which I mean educationally great) arguments when students disagree on the meaning of the term they are both trying to define.