Fertilization is the process in sexual reproduction where a sperm cell (male gamete) fuses with an egg cell (female gamete) to form a single cell called a zygote. This process occurs in both animals and plants, allowing for the genetic material from both parents to combine and create a new, genetically unique individual. The zygote then undergoes multiple rounds of cell division and differentiation, eventually developing into a fully-formed organism. Fertilization is a crucial step in the life cycle of sexually reproducing species, as it ensures the continuation of their genetic lineage and promotes genetic diversity within a population.