It's definition of{a word}. "for" is understandable, but mostly it's "of". According to Google Ngrams, "definition for a word" was virtually unknown until 1920, and it's still used only in a small minority of cases.
You are right. 'For' seemed OK to me when I posted my response, but a subsequent search found no citations in the BNC or COCA. I am clearly in a much smaller minority than I thought.