冬天 (dōngtiān) is a noun meaning "winter." It is the most common everyday word for the winter season in Chinese, used in both casual conversation and more formal contexts to refer to the coldest time of year.
北京的冬天特别冷,经常下雪。
Běijīng de dōngtiān tèbié lěng, jīngcháng xià xuě.
Winter in Beijing is especially cold, and it often snows.
冬天到了,你记得多穿点衣服。
Dōngtiān dào le, nǐ jìde duō chuān diǎn yīfu.
Winter is here — remember to wear more clothes.
我最喜欢冬天在家喝热汤。
Wǒ zuì xǐhuān dōngtiān zài jiā hē rè tāng.
I love drinking hot soup at home in winter the most.