Anacrusis

Anacrusis, in music or poetry, is a word for the lead-in syllables or notes that precede the first full measure.

In the Star Spangled Banner, the word Oh in the first line is in anacrusis in both the music and the anapestic meter of the poem:

x    /    x   x   /    x   x   /     x  x  /   
Oh, say, can you see, by the dawn's early light. . .

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Anacrusis is also the name of a progressive metal band.