focussed

• focused

focussed (comparative more focussed, superlative most focussed)

1. (Britain) Alternative spelling of focused

• unfocussed

focussed

1. (Britain) simple past tense and past participle of focus

• The spelling focused is much more common in the US but also more common in the UK and Australia.
• It violates the UK spelling rule that calls for a double consonant whenever a short vowel sound precedes a suffix beginning with a vowel, perhaps to avoid looking like cussed.
• In UK English, such a consonant must traditionally be doubled even when the short preceding vowel sound is not stressed, e.g. travelled, where US spelling traditionally accepts only traveled (but travelled is sometimes seen in the US and especially programmed is very common). Such consonants are traditionally doubled according to US English spelling rules only when stressed, e.g. called, referred.
• So the spelling focussed is the traditional UK spelling and focused (and focusing) is an exception but so common that the online Cambridge and Oxford dictionaries list it first. The usually non-prescriptive Longman dictionary surprisingly gives only the traditional UK spelling and claims that focused is only US English.
• The traditional UK spelling focussed is nowadays more common in Canada and New Zealand than in the UK.