Once a popular unit
of time measurement,
a fortnight is merely two weeks
Sometime around the turn of one of the last centuries it fell out of general use in USA around colonial times. Occasionally it pops up in intellectual (or, more often, attempts at being intellectual) references such as those referring to America's former President Clinton having "fortnight politics" since that's about how long any of his viewpoints or interests seemed to last.
However, it enjoys much more frequent usage in the rest of the world, including much of Europe, India and Australia.
But where did such a word come from? Why doesn't Webster1913 even know about it?
Old English had a word "feowertyne niht" which literally meant "fourteen nights" around the year 1000. Dropping out all of the strange letters, we get "fortnight". Hence, two weeks.
But it's not quite that simple. Another Xnight that has come and gone throughout history is "sennight" which popped up in Shakespearean times which came from the Old English "seofon niht". Also, to add to the confusion about fortnight's origin, there is the occasionally-mentioned idea that it originates from the period of time a fortified city could operate completely without outside intervention. This is not based in any sort of fact, of course, but nevertheless pops up here and there undaunted.
And now you know.