The term 'grounds for grounding' can be summarized as the reasons why someone or something is grounded. This term is often used in aviation, where it refers to the reasons why an aircraft is not allowed to take off or land. The four-letter solution to this crossword is 'SASS, ' which stands for Safety Assessment System. This system is used by the Federal Aviation Administration (FAA) to assess the safety of aircraft and determine if they are fit to fly.