A fortnight is two weeks. Fourteen nights, which is exactly where the word comes from.
It is thoroughly British and its absence from American English surprises people on both sides: Americans say two weeks, and have no single word for it at all. Britain uses it constantly for holidays, pay and appointments — a fortnight in Spain, paid fortnightly, see you in a fortnight. The adjective and adverb are fortnightly, meaning every two weeks. Sennight, the matching word for seven nights, died out centuries ago.
1nounfortnightA period of two weeks.
“They were away for a fortnight.”
Old English
fēowertīene niht
Middle English
fourtenyght
English, today
fortnight
From Middle English fourtenyght, fourtene nyght, from Old English fēowertīene niht (literally “fourteen nights"; the Anglo-Saxons counted by nights), equivalent to fourteen + night. Compare sennight.
Etymology from Wiktionary, CC BY-SA