The Collaborative International Dictionary of English v.0.48:

Seventieth \Sev"en*ti*eth\, a. [AS. hund-seofontigo[eth]a.]
   1. Next in order after the sixty-ninth; as, a man in the
      seventieth year of his age.
      [1913 Webster]

   2. Constituting or being one of seventy equal parts.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Seventieth \Sev"en*ti*eth\, n.
   1. One next in order after the sixty-ninth.
      [1913 Webster]

   2. The quotient of a unit divided by seventy; one of seventy
      equal parts or fractions.
      [1913 Webster]

WordNet (r) 3.0 (2006):

seventieth
    adj 1: the ordinal number of seventy in counting order [syn:
           seventieth, 70th]
    n 1: position 70 in a countable series of things