A person or thing that defines something, such as a dictionary that explains word meanings.
From define plus -er (agent noun suffix). The -er suffix creates nouns for people or things that perform an action.
A 'definer' could be a person, a dictionary, or even a concept—like how the Rosetta Stone was a crucial definer of Ancient Egyptian language. It's a word that captures the role of anything that sets boundaries or clarifies meaning.
Complete word intelligence in one call. Free tier — 50 lookups/day.