STAMINA is a 7 letter word that starts with S.

1. \Stam"i*na\, n. pl.
   See {Stamen}.
   
   

2. \Stam"i*na\, n. pl.
   1. The fixed, firm part of a body, which supports it or gives
      it strength and solidity; as, the bones are the stamina of
      animal bodies; the ligneous parts of trees are the stamina
      which constitute their strength.
   
   2. Whatever constitutes the principal strength or support of
      anything; power of endurance; backbone; vigor; as, the
      stamina of a constitution or of life; the stamina of a
      State.
   
            He succeeded to great captains who had sapped the
            whole stamina and resistance of the contest. --De
                                                  Quincey.