What does an anacrusis refer to in music?

An anacrusis refers specifically to the notes that occur before the first strong beat of a measure and often lead into the main part of a musical phrase. These notes are generally unstressed, creating a sense of anticipation and flowing smoothly into the downbeat. This technique is most commonly used to enhance musical phrases, adding a dynamic feel by allowing the music to "lean" into the first full measure.

The other choices focus on different musical elements. For instance, the first option talks about tempo, which relates to the speed at which music is played rather than the structure of phrases. The third option, the climax of a musical piece, pertains to the peak of emotional intensity or tension, unrelated to the concept of anacrusis. Lastly, solo passages refer to sections featuring a solo instrument or voice, which also does not align with the definition of an anacrusis. Therefore, identifying unstressed notes at the beginning of a phrase is what accurately describes an anacrusis.