What is the difference between `defence’ and `defense’?

 We know that when it comes to spelling, there is some difference between American and British English. This pair is another example. The British spell it `defence’ and the Americans spell it `defense’. Donald Rumsfeld, for example, is the `Secretary of Defense’. There are many pairs of words in English which are spelt both with a `c’ and an `s’. For example, advice, advise and practice, practise. In the two examples, the noun form of the word is spelt with a `c’ and the verb form is spelt with an `s’.

Source: The Hindu ‘Know Your English’ Series, December 12, 2005