The Collaborative International Dictionary of English v.0.48:

Twentieth \Twen"ti*eth\, n.
   1. The next in order after the nineteenth; one coming after
      nineteen others.
      [1913 Webster]

   2. The quotient of a unit divided by twenty; one of twenty
      equal parts of one whole.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Twentieth \Twen"ti*eth\, a. [From Twenty; cf. AS.
   twentigo[eth]a. See Twenty.]
   1. Next in order after the nineteenth; tenth after the tenth;
      coming after nineteen others; -- the ordinal of twenty.
      [1913 Webster]

   2. Constituting, or being, one of twenty equal parts into
      which anything is divided.
      [1913 Webster]

WordNet (r) 3.0 (2006):

twentieth
    adj 1: coming next after the nineteenth in position [syn:
           twentieth, 20th]
    n 1: position 20 in a countable series of things