“Word Is” lets users write down their own definitions of what words mean to them and then rate which ones they like best.

Some of the definitions can be a bit esoteric, so I wouldn’t recommend it as a place for English Language Learners to study for vocabulary development (though I’m sure they would pick-up some useful info). However, it would be a nice place for students to write their own definitions.

I’m adding it to The Best Places Where Students Can Write For An “Authentic Audience.”