What is a "Missed Approach"?

In the past, if pilots could not see either the approach lights or runway after descending to a certain height called the MDA, or “Minimum Descent Altitude,” they had to execute a missed approach by adding power, climbing back to a higher altitude and trying the approach again, or diverting to an alternate airport. With today’s advanced Category III or “Cat III” landing parameters, aircraft can be automatically guided all the way down to a safe landing on the runway in “zero-zero” conditions (no ceiling/no visibility) without the pilots ever seeing the ground.