Webster is right, but not terribly thorough.

Alexandrine is verse/poetry in iambic hexameter (six feet, stress on the first beat). More common in French than English, it's usually introduced into mostly iambic pentameter verse for variety sake. A Spenserian Stanza has 8 lines of pentameter followed by a line of hexameter, for example. For an example, see the Pope verse from Webster 1913 below.