coldest period; depth of winter
Refers to the coldest part of winter, often used in meteorological contexts or to describe severe winter conditions.
During the coldest period, temperatures drop below freezing.
It's better to plan your mountain climbing to avoid the depth of winter.
Compound of 厳 (gen, 'severe') + 冬 (tō, 'winter') + 期 (ki, 'period').