The Bedford Glossary of Critical and Literary Terms - Ross Murfin 2018
Anacrusis: One or more extra unstressed syllables at the beginning of a line of verse that are not counted as part of the meter.
EXAMPLE: Samuel Taylor Coleridge’s “The Rime of the Ancient Mariner” (1798) is written primarily in four-line stanzas that alternate tetrameter and trimeter lines. The “and” in the first iambic tetrameter line of the following stanza exemplifies anacrusis:
And th̆e báy wăs whі́te wı̇̆th sі́len̆t liǵht,
Till rising from the same,
Full many shapes, that shadows were,
In crimson colors came.