Source : Webster's Revised Unabridged Dictionary (1913)

Simmer \Sim"mer\, v. i. [imp. & p. p. {Simmered}; p. pr. & vb.
   n. {Simmering}.] [Prov. E. also simper; -- an onomatopoetic
   word.]
   To boil gently, or with a gentle hissing; to begin to boil.

         I simmer as liquor doth on the fire before it beginneth
         to boil.                                 --Palsgrave.

Simmer \Sim"mer\, v. t.
   To cause to boil gently; to cook in liquid heated almost or
   just to the boiling point.

Source : WordNet®

simmer
     n : temperature just below the boiling point; "the stew remained
         at a simmer for hours"
     v : boil slowly at low temperature; "simmer the sauce";
         "simmering water"