A line of six iambic feet, often used to mark a conclusion in a work which is in heroic couplets: Alexander Pope in his Essay on Criticism satirised this technique:
Then, at the last and only couplet fraught
With some unmeaning thing they call a thought,
A needless Alexandrine ends the song,
That like a wounded snake, drags its slow length along.
The final line of that extract is of course itself an alexandrine. Spenser used an alexandrine to end his modified form of ottava rima. The same word is used to describe a line of twelve syllables which is the dominant form of French verse.