twelfth / twelf[unvoicedth]/ • ordinal number constituting number twelve in a sequence; 12th: the twelfth of November his twelfth birthday the twelfth in a series of essays. ∎  (a twelfth/one twelfth) each of twelve equal parts into which something is or may be divided. ∎  the twelfth grade of a school. ∎  Mus. an interval or chord spanning an octave and a fifth in the diatonic scale, or a note separated from another by this interval. ∎  (the Twelfth) July 12, celebrated by upholders of Protestant supremacy in Ireland as the anniversary of William III's victory over James II at the Battle of the Boyne.DERIVATIVES: twelfth·ly adv.twelve·fold / ˈtwel(v)ˌfōld/ adj. & adv.

twelfth constituting number twelve in a sequence. The phrase The Twelfth denotes both 12 July, celebrated by upholders of Protestant supremacy in Ireland as the anniversary of William III's victory over James II at the Battle of the Boyne, and (in the UK) 12 August, the day on which the grouse-shooting season begins.
twelfth man in cricket, a player nominated to act as a reserve in a game, typically carrying out duties such as fielding as a substitute and taking out drinks.
Twelfth Night 6 January, the feast of the Epiphany. Strictly, this denotes the evening of 5 January, the eve of the Epiphany and formerly the twelfth and last day of Christmas festivities.

twelfth. Org. stop of the mutation kind. Length and pitch 2 2/3’ sounding an octave and a 5th (i.e. a 12th) above normal.