caster sugar vs castor sugar

caster sugar and castor sugar both are nouns.

Word	Noun	Adjective	Verb	Adverb
caster sugar	Yes	No	No	No
castor sugar	Yes	No	No	No

As nouns, caster sugar and castor sugar are synonyms defined as:

caster sugar and castor sugar: very finely granulated sugar that was formerly sprinkled from a castor

caster sugar (noun)	castor sugar (noun)
very finely granulated sugar that was formerly sprinkled from a castor	very finely granulated sugar that was formerly sprinkled from a castor