Grain And Cereal

Cereal
(Countable) A type of grass (such as wheat, rice or oats) cultivated for its edible grains.

Grain
(Uncountable) The harvested seeds of various grass food crops eg: wheat, corn, barley.
‘We stored a thousand tons of grain for the winter.’

Cereal
(Uncountable) The grains of such a grass.

Grain
(Uncountable) Similar seeds from any food crop, eg buckwheat, amaranth, quinoa.

Cereal
(Uncountable) Breakfast cereal.
‘Would you like some cereal?’; ‘Which cereal would you like for breakfast?’

Grain
(Countable) A single seed of grass food crops.
‘A grain of wheat’; ‘grains of oat’;