seventh

Concise Oxford English Dictionary © 2008 Oxford University Press:
seventh/ˈsevnθ/

▶ordinal number

• 1 constituting number seven in a sequence; 7th.

• 2 (a seventh/one seventh) each of seven equal parts into which something is or may be divided.

• 3 Music an interval spanning seven consecutive notes in a diatonic scale.

  ■ the note which is higher by this interval than the tonic of a diatonic scale or root of a chord.

  ■ a chord in which the seventh note of the scale forms an important component.

– derivatives
seventhly adverb.