Practice or Practise? Both sound the same: /ˈpræktɪs/, but one is a noun and the other is a verb. The Americans don't make it better by spelling both the same way: practice.
The way I remember it is very simple. Noun comes before Verb in our alphabet, and similarly, C comes before S; therefore, the N corresponds to the C, and the V corresponds to the S; in other words, practice is the noun, and practise is the verb!