Answer: Literary Device
A literary device is essentially a narrative technique. Literary devices are strategies that allow an author to enrich his text. These tend to make writing more vivid, complex, emotional and energetic. Moreover, they allow the author to fully display his imagination and creativity. In this series of poems, Wordsworth used "Lucy" as a literary device. It is unlikely that the girl existed. Instead, she is a device that is used in the poem in order to explore Romantic ideas of love, beauty and grief.