What is a passenger disturbance level?
The FAA has categorized disturbances into four levels:


  Level 1: Disruptive behavior - suspicious or threatening
    Level 2: Physically abusive behavior
    Level 3: Life-threatening behavior
    Level 4: Attempted or actual breach of the flight deck 


These are also ICAO standard classifications from the restricted Doc 9811, according to IATA. I'm not sure who came up with them first. That IATA document also provides sample things that might be considered to be each level.