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?
Editors. "Anacrusis". The Literary Encyclopedia. First published 01 November 2001
[http://www.litencyc.com/php/stopics.php?rec=true&UID=41, accessed 17 January 2017.]