The phrase "to define the period" means to establish or determine the unique characteristics, features, or important events that make a specific time period distinct from others. Full definition
Their ability to raise their revenues or rents is constrained by the fact that their leases are locked in for defined periods of time. (moneysense.ca)
This type of is best suited for defined periods of need - e.g., during the years when you're concerned about providing for your children and their education. (investopedia.com)
The impulse wave defines the period in a bullish trend where the asset makes a sustained move toward higher prices. (profitconfidential.com)