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]

WordNet (r) 3.0 (2006):

somersault
    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]
    v 1: do a somersault

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "somersault":
   be lost, capsizal, capsize, culbute, founder, go down, keel,
   keel over, overset, overthrow, overturn, pitchpole, revolution,
   scuttle, sink, somerset, spill, subversion, turn over, turn turtle,
   turnover, upset, upset the boat, upturn