The insertion of one or more unstressed syllables at the beginning of a line where the poetical metre would normally demand a stressed syllable. For example, the “And” which opens the second line of Blake's “The Tiger” is logically an unstressed word but stands in a place where the rhythm requires a stress:
When the stars threw down their spears,
And watered heaven with their tears,
Did he smile his work to see?
Did he who made the Lamb make thee?