There are of course multiple definitions of ‘definition,’ and while we cannot know exactly which one was in mind as ‘definition’ made it onto the banned words list, we can only suppose it may at least in part be due to a fawning love for its opposite, obfuscation.
Sources of the banned words list:
https://pen.org/banned-words-list/
https://newrepublic.com/post/193395/agriculture-department-ban-words-safe-drinking-water