There are thousands of species of cactus and most of them have impressive flowers. This flower is from the genus Epiphyllum. Here is a short explanation of this genus from Wikipedia: Epiphyllum.
Epiphyllum (/ˌɛpᵻˈfɪləm/; "upon the leaf" in Greek) is a genus of 19 species of epiphytic plants in the cactus family (Cactaceae), native to Central America. Common names for these species include climbing cacti, orchid cacti and leaf cacti, though the latter also refers to the genus Pereskia.Many of these lovely flowers have been created through hybridization.