sixth

Concise Oxford English Dictionary © 2008 Oxford University Press:
sixth/sɪksθ/

▶ordinal number

• 1 constituting number six in a sequence; 6th.

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

• 3 Music an interval spanning six consecutive notes in a diatonic major or minor scale, e.g. C to A (major sixth) or A to F (minor sixth).

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

• 4 Brit. the sixth form of a school.

– derivatives
sixthly adverb.