The Collaborative International Dictionary of English v.0.48:

Sixtieth \Six"ti*eth\, a. [As. sixtiogo[eth]a, sixtigo[eth]a.]
   1. Next in order after the fifty-ninth.
      [1913 Webster]

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

The Collaborative International Dictionary of English v.0.48:

Sixtieth \Six"ti*eth\, n.
   1. The quotient of a unit divided by sixty; one of sixty
      equal parts forming a whole.
      [1913 Webster]

   2. The next in order after the fifty-ninth; the tenth after
      the fiftieth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sixtieth
    adj 1: the ordinal number of sixty in counting order [syn:
           sixtieth, 60th]
    n 1: position 60 in a countable series of things
    2: one part in sixty equal parts [syn: one-sixtieth,
       sixtieth]