Named after the ever-so-heroic Alexander the Great.
It is a line of verse made up of six iambs.
Alexandrine is a verse form that is the leading measure in French poetry.
It consists of a line of 12 syllables with major stresses on the 6th syllable and on the last syllable.
The term probably derived from the early use of the verse in the French Roman d’Alexandre, a collection of romances that was compiled in the 12th century about the adventures of Alexander the Great.
In English versification, the alexandrine is also called Iambic hexameter.
French alexandrine is syllabic, the English is accentual-syllabic.
In English it is used, eg: as the last line of Spenserian stanza or as a variant in a poem of heroic couplets, rarely in a whole work.
Eg: An Essay On Criticism by Alexander Pope, To Some Birds Flown Away by Victor Hugo.