wintertime; winter season
Refers to the winter period, often with a nuance of the cold or harsh conditions typical of winter.
In winter, roads freeze easily, so you need to be careful when driving.
In this region, a lot of snow falls during the winter season.
During the winter, the number of tourists decreases.
Many people fall ill during the winter.
Please be careful about the dryness during the winter.
冬季 is a formal, often official term for the winter season (as in 冬季オリンピック), whereas 冬場 is more everyday and used to describe conditions or business trends during that time.
夏場 is the parallel term for the summer period, contrasting directly with 冬場's focus on the winter period.
Compound of 冬 (ふゆ, 'winter') and 場 (ば, 'place' or 'time'), literally 'winter time/season'.