Hi Andrei, the term fortnight is quite commonly used in Australia, but I think not in American English. We would say 'I'm going camping/on holiday/away for a fortnight in March/May/July' etc and everyone would know you meant two weeks.

It is commonly used in British English, both spoken and written, for a period of two weeks. It is also used in some other English speaking countries, but, it seems, is regarded as archaic in the USA. It is derived from 'fourteen nights'

"Fortnight" is a length of time, meaning two weeks. As Bramwell says, it comes from 'fourteen nights. You can use it any time you want to refer to this period of time. You can say, for example. "I'm going to Spain in a fortnight" ( meaning that you're leaving in two weeks' time) or "I'm going to Spain for a fortnight" (meaning that you're going to spend two weeks there). You can say "I visit my parents once a fortnight", meaning twice a month. You can say "I've got a fortnight's leave", meaning that you've got two weeks off work. And so on. The answers that Cole and Leslie have given apply only to the American perspective.

Technically it means 2 weeks or 14 days.

Edit: In American English it is a very dated term. I would suggest that you never really use it, unless you are trying to be funny, or are translating older texts from other languages.