Practice - practise

Practice and practise are two often-confused homophones, closely related in meaning.

• A practice is a noun – something that you do, e.g. “there is a football practice tonight”; “I did 5 hours piano practice yesterday”; “my usual practice is to warm up for five minutes first”; “she is in General Practice”. (If you can say “a practice” in your sentence, then it is practice.)

• To practise is a verb – to do something, e.g. “I practised my vocabulary last night”; “he practised his speech in front of a mirror.”

There is a similar variation with 'advice' or 'advise'. These two are not homophones, so you can hear which you mean. "The Supervisor advised his student to change the focus of his Thesis", it may seem obvious to a British mother-tongue speaker of English, is better than "The Supervisor adviced his student to change the focus of his Thesis". The sound of the first sibilant is self-evidently voiced, like a '-z-', while that of the second is unvoiced.

Note: In American English, neither of the pairs practice/practise nor advice/advise exists.