fertilisation of an egg (a secondary oocyte) by more than one sperm (spermatozoon). In in vitro fertilisation, more common if eggs are recovered that are either immature or overly mature. Evident later, with the appearance of more than two pronuclei. In natural conditions, a cause of a polyploid state in the embryo. Another name for it is polyspermic fertilisation.

A visible 'blob' (or vacuole) -- there are usually two of them -- inside an egg that has undergone fertilisation or activation by other means, and enclosing the chromosomes from the oocyte (the female pronucleus) and from the sperm (the male pronucleus). Plural: pronuclei. If three pronuclei are observed, the chances are that two sperm have entered the egg (polyspermy).