The stated number of days after expiry within which a late renewal still counts as a renewal.
A grace window is the period after a contract's end date, often thirty days, within which a renewal is counted as continuous. Renewals inside it are renewals with a late flag; renewals after it are a loss followed by new business, because the customer was without the service for that long. The window is stated once and applied to every contract, which is what makes renewal rates comparable across periods.
A stated number of days after a contract ends within which a renewal still counts as continuous, applied to every contract alike.
With a thirty-day window, 92 of 100 contracts count as renewed. Counting three that renewed after ninety days would make it 95. The three are losses won back.
No stated window. Late renewals are counted or not depending on who prepares the figure.