polyspermy

n. fertilization of an egg by several sperm cells

In biology, polyspermy describes an egg that has been fertilized by more than one sperm.  Diploid organisms normally contain two copies of each chromosome, one from each parent. The cell resulting from polyspermy, on the other hand, contains three or more copies of each chromosome -- one from the egg and one each from multiple sperm. Usually, the result is an inviable zygote. This may occur because sperm have become too efficient at reaching and fertilizing eggs due to the selective pressures of sperm competition. This is often deleterious to the female, in other words the male-male competition among sperm spills over to create sexual conflict.
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(n.)
Fullness of sperm, or seed; the passage of more than one spermatozoon into the vitellus in the impregnation of the ovum.
  

Passage of more than one sperm cell into a single ovum at fertilization.