The fusion of ovum and sperm is called _________.

Fertilization:

1. Fertilization is the process where sperm (male gamete) fuses with the ovum (female gametes) to form a single cell called a zygote.
2. In humans, the fertilization process most often occurs when sexual intercourse takes place during a woman's fertile or ovulation period.

Final answer: The fusion of ovum and sperm is called fertilization.