1. [n]  position 60 in a countable series of things
2. [adj]  the ordinal number of sixty in counting order

SIXTIETH is a 8 letter word that starts with S.

1. \Six"ti*eth\, a. [As. sixtiogo[eth]a, sixtigo[eth]a.]
   1. Next in order after the fifty-ninth.
   
   2. Constituting or being one one of sixty equal parts into
      which anything is divided.
   
   

2. \Six"ti*eth\, n.
   1. The quotient of a unit divided by sixty; one of sixty
      equal parts forming a whole.
   
   2. The next in order after the fifty-ninth; the tenth after
      the fiftieth.