In syllabic verse, such as that used in French literature, an alexandrine is a line of twelve syllables. There is some controversy as to the origin of the name, but most probably it is derived from a collection of Alexandrine romances, collected in the 12th century, of which Alexander the Great was the hero, written entirely in alexandrines.
If you remember your French lessons, you know that an alexandrine is separated by two half-lines 6 ', separated by a pause called hyphenation.
Through the harsh cadence of a rugged line
A noble error, and but seldom made,
When poets are by too much force betrayed.
Thy generous fruits, though gathered ere their prime,
Still showed a quickness; and maturing time
But mellows what we write to the dull sweets of rhyme.