The Collaborative International Dictionary of English v.0.48:

Somersault \Som"er*sault\, Somerset \Som"er*set\, n. [F.
   soubresaut a jump, leap, OF. soubresault, It. soprassalto an
   overleap, fr. L. supra over + saltus a leap, fr. salire to
   leap; or the French may be from Sp. sobresalto a sudden
   asault, a surprise. See Supra, and Salient.]
   A leap in which a person turns his heels over his head and
   lights upon his feet; a turning end over end. [Written also
   summersault, sommerset, summerset, etc.] "The vaulter's
   sombersalts." --Donne.
   [1913 Webster]

         Now I'll only
         Make him break his neck in doing a sommerset. --Beau. &
                                                  Fl.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Summersault \Sum"mer*sault\, Summerset \Sum"mer*set\, n.
   See Somersault, Somerset.
   [1913 Webster]

WordNet (r) 3.0 (2006):

summersault
    n 1: an acrobatic feat in which the feet roll over the head
         (either forward or backward) and return [syn: somersault,
         somerset, summersault, summerset, somersaulting,
         flip]