Students learn how to create good definitions for grade-level vocabulary words by thinking about how they would explain those words to a space alien who barely understands English.
First, students read through a list of vocabulary words and mark the ones with which they are unfamiliar. Then, they are to create sentences using words they already know, look up the dictionary definitions of words they don't know, create their own definitions and pronunciation guides, and finally draw pictures or symbols to remind them of the meanings of the words.
This packet includes a list of words that fifth-graders should know according to Flocabulary. I am not related to them in any way, but I thought it was a good list. The activities, however, would be just as suited for older students as well: just substitute your own vocabulary words.