The first line of Dylan Thomas’s poem "Do Not Go Gentle Into That Good Night" is called the first refrain.
In poetic terms, a refrain is a line or group of lines that is repeated at intervals throughout the poem. In this poem, the repetition of the line “Do not go gentle into that good night” serves as a refrain, emphasizing the poem's themes of struggle and resistance against death.
Therefore, the correct answer is first refrain.