The nucleus of the ovum or sperm after fertilization but before they fuse to form the nucleus of the zygote WordNet 3.0 & Open English WordNet

A pronucleus denotes the nucleus found in either a sperm or egg cell during the process of fertilization. Wikipedia

The nucleus of either the ovum or the spermatozoon following fertilization. Thus, in the fertilized ovum, there are two pronuclei, one originating from the ovum, the other from the spermatozoon that brought about fertilization; they approach each oth Wikidata

Either of the two haploid nuclei (of a sperm and ovum) that fuse during fertilization. Wiktionary