The full form of AM is Ante Meridiem, which is a Latin term used to indicate a 12-hour clock system before noon. It means "before midday". The full form of PM is Post Meridiem, which is another Latin term used to indicate the 12-hour clock system after noon. It means "after midday". The 12-hour clock system splits a days time cycle into two parts, with AM representing the first period from midnight to noon, and PM representing the second period from noon to midnight.