Happening or appearing once in or every two weeks.(adjective)
Once in a fortnight.(adverb)
A publication issued once every two weeks.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "fortnightly" in a sentence
"You should look to use the word "fortnightly" at least once every two weeks."
"About 2% of my excitement in working for the Guardian has to do with seeing my "z's" replaced with "s's" and being able to use the word "fortnightly" more often and completely appropriately."
"She was treated with the atypical anti-psychotic medication — administered in fortnightly injections so it is not left up to a patient to take the medicine every day."