What is a human? What is knowledge? What is virtue? What is justice?
We have known for 2500 years that some words are very difficult to define in such a way that every single edge case is handled, it is complete, and short. The most famous example, 2500 years ago, was an academy defined human as a featherless biped. The next day someone released a plucked chicken.
For those words that are very difficult to define we develop criteria and gradually alter the criteria as time goes on, mostly based on the idea of ordinary language.