Final answer:
The term for the fusion of gametes during fertilization is zygote formation, which results in a diploid zygote.
Step-by-step explanation:
The term for the fusion of gametes during fertilization to form a diploid cell is zygote formation. In sexual reproduction, gametes, which are haploid cells containing only one set of chromosomes, merge to create a diploid zygote that includes both sets of chromosomes from the parents. This process differs from mitosis and meiosis, with meiosis being the process responsible for the creation of haploid gametes prior to fertilization.