Final answer:
In the poem "Janet waking," the word "But" is an example of a shift word, which indicates a change in the direction or tone of the poem.
Step-by-step explanation:
In the context of poetry, a shift word is a word that signifies a change in the speaker's thought process or the direction of the poem. In the poem "Janet waking," the word "But" serves as a shift word, marking a transition in tone, mood, or perspective, much like 'then' or 'yet' might in other works. This word signals a contrast or an exception to what has previously been mentioned, and it often precedes a turning point in the poem's narrative or argument.