Word of the Day:
the process of varying a training program at regular time intervals to bring about optimal gains in physical performance; training is divided into segments of time, or periods, annually or in mixed phases over the course of a yearly cycle or in mixed phases of 8 to 12 weeks.
Planning periods or cycles within an overall program promotes long-term training and performance including variations in training specificity, intensity, and volume.
Basically, it prevents boredom, overtraining, and plateaus so you are always challenging your body.
During the off-season, most triathletes plan their periodization for the next year.
"I TRI; therefore, I am."