I’ll tell the definition. Not the pilots, not the companies, not the passengers. Proper staffing means when a crew gets stuck at an outstation for wx or mx or someone gets sick/emergency we have someone available to call (aka reserve) without cold calling everyone on their day off which many times results in a cancelation. It means they don’t need to go back and manually remove minutes from each flight for pushback to make pilots legal for that extra block time because they have no one available to replace them. It means not violating policy to force people into doing flying that they aren’t required to do because....you guessed it..they don’t have any available pilots to replace them.
Proper staffing would be when you start the day with a 4 hour mx delay they look at your schedule and say “hey we are going to give your next turn to a reserve crew to keep everything on time”.

They get away with it all because they code it as anything under the sun other than not having available crew anytime they can stretch it.