diagonalisation vs British spelling

diagonalisation and British spelling both are nouns.

Word	Noun	Adjective	Verb	Adverb
diagonalisation	Yes	No	No	No
British spelling	Yes	No	No	No

diagonalisation (noun)	British spelling (noun)
changing a square matrix to diagonal form (with all non-zero elements on the principal diagonal)	English orthography used in the UK, Ireland and most of the Commonwealth (characterized by 'analyse', 'centre' and 'useable')