twelfth

Concise Oxford English Dictionary © 2008 Oxford University Press:
twelfth /twɛlfθ, twɛlθ/

▶ordinal number

• 1 constituting number twelve in a sequence; 12th.

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

• 3 Music an interval or chord spanning an octave and a fifth in the diatonic scale, or a note separated from another by this interval.

• 4 (the (Glorious) Twelfth) (in the UK) 12 August, the day on which the grouse-shooting season begins.

– derivatives
twelfthly adverb,
twelvefold adjective & adverb.