一下子 (yī xià zi) means “in a short while; all at once; all of a sudden”. It is an HSK 5 Chinese word in the HSK 3.0 vocabulary.
Practice each character:
天气一下子变冷了。
Tiānqì yīxiàzi biàn lěng le.
The weather suddenly turned cold.
Updated: 2026-07-18