What is clean? Is you definition of clean based on how it looks? Does something seem more "clean" if it smells clean or fresh? Does adding a fresh scent perfume to a cleanser make it work better? What smells do you associate with clean? - lemon? pine? alcohol? ammonia? Some of these smells remind us of the clean smell of a doctor's office, others of the fresh air outdoors. Take a look at products in the market, and check for advertisements which include scents as part of the criteria for clean.
When scientists share and compare data, they need to agree on definitions which will allow them to communicate and compare effectively. These definitions work, or "operate" effectively to allow fair comparisons and communication - they are often called "operational definitions." Can you write an operational definition for "clean?"