Period during which a debt instrument cannot technically be repaid or called.
If the loan or bond is repaid, then this involves the payment of a very expensive Make-Whole Premium.
Most often this is a construct seen in high yield bonds, with a typical construct of a 5NC2 (5 year maturity, non-call for the first 2 years) or 7NC3 (7 year maturity, non-call for the first 3 years); In case of floating rate bonds, the non-call period is usually of just 1 year (5NC1 or 7NC1)
See also: Call Premium, Make-Whole Premium