Cantonese wide flat rice noodles. The English pronunciation chow fun actually corresponds to the Chinese 炒粉 (cháau fán in Cantonese) which is short for 沙河粉 (sā hòh fán), where 沙河 is the town where these noodles are believed to have originated and 粉 (fán) is rice noodles.
Ordering
碟 is the classifier for plate, so you'd order 一碟河粉 (in Cantonese, yāt dihp hòh fán) one plate of chow fun.