Return Period (T)
is the calculated number of years within which a hydrometric event of a certain intensity or magnitude may be expected to occur
Frequency
is defined as the rate of occurrence, or that period of time within which an event of specific magnitude may be expected to occur
Probability (P)
is the odds that an event of a certain intensity or magnitude will occur within a certain period of time.
instantaneous maximum peak flow
peak flow at any one moment during a day.