Source : Webster's Revised Unabridged Dictionary (1913)

Superfecundation \Su`per*fec`un*da"tion\, n. (Physiol.)
   Fertilization of two ova, at the same menstruation, by two
   different acts of coition.

Source : WordNet®

superfecundation
     n : fertilization of two or more ova released during the same
         menstrual cycle by sperm from separate acts of coitus
         (especially by different males)