Currently used definitions of the cup:

The US customary cup (236.6 mL) is 8 US customary fluid ounces. The US customary fluid ounce (29.6 mL) is 1/16 of a US fluid pint.

The US legal cup (240 mL) is 8 US nutritional fluid ounces. The US nutritional fluid ounce is 30 mL.

The metric cup is 250 mL

Historically used definitions of the cup:

Ths British cup (284.1 mL) is 10 imperial fluid ounces. The imperial fluid ounce (28.4 mL) is 1/20 of an imperial fluid pint

The Canadian cup (227.3 mL) is 8 imperial fluid ounces