Meter

Definition: The rhythmic structure of a verse using stressed and unstressed syllables

Example:
The As SYR..|..ian came DOWN..|..like the WOLF..|..on the FOLD,
........1.......................2..........................3....................4
And his CO..|..horts were GLEAM..|..ing in PUR..|..ple and GOLD; 

Explanation: To make the poem more catchy and rhythmic when the readers read aloud.