tumor vs tumour

tumor and tumour both are nouns.

Word	Noun	Adjective	Verb	Adverb
tumor	Yes	No	No	No
tumour	Yes	No	No	No

As nouns, tumor and tumour are synonyms defined as:

Other synonyms of tumor include neoplasm.

tumor (noun)	tumour (noun)
an abnormal new mass of tissue that serves no purpose	an abnormal new mass of tissue that serves no purpose