What does 'ottava' refer to in music terminology?

'Ottava' in music terminology specifically refers to the instruction to perform notes an octave higher or lower than notated. When used in sheet music, ‘ottava’ indicates that the player should transpose the pitch to an octave above (ottava alta) or below (ottava bassa) the written note. This concept is crucial in achieving certain sound textures or to accommodate the range of specific instruments.

Understanding the other options can help clarify why 'adding an octave' is the most relevant choice. The term does not pertain to tempo changes or articulation marks—hence, it does not refer to returning to speed, slowing down, or marking something clearly. Each of these actions relates to different musical directives that govern how music should be played regarding pace or clarity, rather than pitch relation, which is central to the definition of 'ottava.'