A fortnight is two weeks — fourteen nights, if you say it the old way. So fortnightly means once every two weeks. A fortnightly magazine comes out every other week; a fortnightly meeting happens every two weeks. You can use it before a noun (a fortnightly report) or after a verb (the paper comes out fortnightly). British English uses this word quite naturally. American English almost never does; Americans say every two weeks, or biweekly — though biweekly is messy, because some people use it for twice a week. In Taiwan you will meet fortnightly in British school books and in the names of magazines.