A fourteener is a line of verse made up of fourteen syllables, most often in iambic heptameter-seven iambic beats in a row. English poets often divide it into two balanced halves, which can make the line sound rolling, spacious, or songlike.
Fourteeners are common in long narrative and ballad-style verse, though they can also appear in lyrical poems when a poet wants a full, flowing line.
Example
The moonlit field grew silver as we walked the road
and heard the winter river turning under stone.