The union of two gametes from opposite sexes to form a zygote; it involves the fusion of nuclei of gametes (karyogamy) and the fusion of cytoplasm (plasmogamy). Typically, each gamete contains a haploid set of chromosomes. Hence, after fusion of the nuclei, the resulting nucleus contains a diploid set of chromosomes.

Several categories are distinguished:

1. Self-fertilization (selfing) - fusion of male and female gametes from the same euploid organism.

2. Cross-fertilization (crossing) - fusion of male and female gametes from different euploid individuals.

3. Double fertilization - in angiosperms, the fusion of one male gamete with the ovum at about the same time as the second male gamete fuses with the female polar nuclei (or secondary nucleus) to form the endosperm.