yī​huí​shì HSK7

one thing

一回事 yī​huí​shì
one thing numeral
a single matter as distinct from another, typically used in contrastive structures

理论一回事实践一回事

lǐ​lùn shì yī​huí​shì, shí​jiàn yòu shì lìng yī​huí​shì.

Theory is one thing, practice is another.

same thing numeral
one and the same thing or matter

These two cases are actually the same thing.

Frequency

Written text
6.09 per million

Details

HSK 3.0
Level 7

Sources

Definitions
CC-CEDICT
Frequency data
BCC Corpus (BLCU)