I drifted onto this web-based dictionary tool that looks interesting, http://www.wordia.com/. I looked at the commercial site from this link and was intrigued by the ability to type in a word and you get not only definitions but also videos that describe or use the word. Contributors can upload their own videos to describe the word, and enter definitions as you would find in a dictionary. Here is a YouTube video of the founder of Wordia explaining the site from 2008, when the site was brand new. However, I was nervous about the site using YouTube videos and Twitter tweets- great for the students but potentially trouble for the teacher.
Good news- there is a Wordia Schools option. Students can create their own pages for words and teachers can choose appropriate terms for their students. Words are available in all subjects. Apparently students can not only add videos of their own but keep track of their own personal dictionary and their progress. The videos are monitored, not available on their public site. I have signed up for an account and hope to create something soon, pertaining to one or more of the human body systems my students are studying now!