British spelling past participle: made aware that something isn't as good as believed; freed from illusion.
Past tense of 'disillusionise' (British form), combining disillusion + -ised (British past tense suffix). Parallels the American 'disillusioned' but with British spelling conventions.
If you see 'disillusionised' in a British novel or newspaper, you're witnessing English spelling history—it's the British form maintaining the '-ise' pattern used for other verbs like 'realise,' 'organise,' and 'emphasise.'
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