Third person singular present tense: causes to lose focus or become blurred (British spelling variant).
From defocus + -es (British spelling third person singular marker). In British English, focus becomes focusses in third person, while American English more commonly uses focuses.
This spelling variant reveals the great vowel divide between American and British English—Brits often double consonants before suffixes (focusses, focussed), while Americans prefer single consonants (focuses, focused), a pattern dating back to Noah Webster's 1828 dictionary reform efforts.
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