The British spelling of diphthongization; the process of converting a vowel into a diphthong.
From 'diphthongise' (British verb) + '-ation'. This spelling variant reflects British English conventions of using '-ise' instead of '-ize' for verb forms and their corresponding noun endings.
You can spot which side of the Atlantic a linguistics paper is from by whether it uses 'diphthongization' or 'diphthongisation'—it's a fun example of how even technical terminology reflects regional English differences!
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