Out of period
Out Of Period (often abbreviated OOP) refers to anything that was not discovered or in general use during the time-frame covered by the SCA. By contrast, anything that falls within the accepted time-frame is said to be "in period" or simply period.
Some items that are OOP are considered by some to be "period by consensus" (not sure of the exact phrasing) as life would be way too difficult without them. This would seem to be a regional use or concept. Many places simply acknowledge them as OOP but use them anyway.