Here's a recipe for schizophrenia: be an Australian author, who for preference writes in UK English, but who is published mostly in the US. I'm well on the way to becoming a walking encyclopaedia of English dialect differences. So let me share some of the madness with practise vs practice.
Practice with a C and practise with an S are two different parts of exactly the same word.
Practice is a noun. In every English speaking country in the world, with one exception, practice is only ever a noun. In that one other country, practice is also a verb.
Everywhere else, practise is always the verb. Hence:
The doctor practises medicine at his practice.
The US lost the S word. So in the US, the doctor practices medicine at his practice. Which to my eye looks horribly wrong.
Just to make it more fun, practise also used to mean to play a trick on someone.
The English practice originates from the Old French practiser, so that the 's' version is the original, and in medical Latin is spelt with a 'z'. It also appears in Greek as praktike. (It's also in Esperanto as praktike!) Since it's in both Latin and Greek, that makes it a very old Indo-European word.
The Oxford English Dictionary gives practice as interchangeable these days, thanks to the US practice of spelling practise as practice. (Confused yet?)
I checked Merriam-Webster's, and it says, to my astonishment, that practise remains acceptable usage in some parts of the US. It doesn't say where, but I guess they mean New England. It also gives practise as meaning to play a joke, in US usage!