denoting a person or thing having the position in a sequence corresponding to the number twelve
one of twelve equal parts of something
in music, an interval of twelve degrees of a diatonic scale, or the combination of two notes at such an interval
(the (Glorious) Twelfth) in Britain, 12 August, the day on which the grouse-shooting season begins
[Old English twelfta, from twelf twelve + ta -th1]