When I was a youngster, and doing dictionary exercises, we were never allowed to define a word by using the root of the word, or any form of the word in the definition. For example, we could not use “important” to define “importantly.”
We must have gone to the same school. Were you allowed to say, "(Word to be defined) is where," or "(Word to be defined) is when"?
Each of those phrasings was prohibited, along with using the root of the word to be defined in the definition.