1. [n]  position 70 in a countable series of things
2. [adj]  the ordinal number of seventy in counting order

SEVENTIETH is a 10 letter word that starts with S.

1. \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.
   
   2. Constituting or being one of seventy equal parts.
   
   

2. \Sev"en*ti*eth\, n.
   1. One next in order after the sixty-ninth.
   
   2. The quotient of a unit divided by seventy; one of seventy
      equal parts or fractions.