Final answer:
The figurative language device used in the line 'Hope is the thing with feathers' is a metaphor, which directly compares hope to a bird with feathers to convey the lightness and resilience of hope.
Step-by-step explanation:
The line "Hope is the thing with feathers" from Emily Dickinson's poem uses a figurative language device known as a metaphor. A metaphor is a direct comparison between two unlike things without using 'like' or 'as'. This poetic line compares the abstract concept of hope to a bird with feathers, implying that hope is light and can lift the spirit. It does not literally mean that hope has feathers but rather suggests that hope has qualities similar to a bird -- for example, the ability to rise above difficulties or remain resilient in the face of challenges.