What is the definition of karyogamy in biology?

Karyogamy in biology is defined as the process where the nuclei of two haploid cells fuse to form a single diploid nucleus, typically occurring as a critical step in sexual reproduction for many organisms, including fungi, protists, and some animals. This event usually follows plasmogamy (fusion of cytoplasm) and results in a diploid cell that may proceed to undergo meiosis or mitosis, depending on the organism.