Heterogenesis, or the production of offspring unlike the parent organisms, is a concept that has several synonyms which can be used interchangeably. These synonyms include: unisexual reproduction, asexual reproduction, parthenogenesis, spontaneous generation, and alternation of generations. Unisexual reproduction refers to the production of offspring by a single parent organism, while asexual reproduction involves the creation of genetically identical offspring from a single parent. Parthenogenesis is a specific type of asexual reproduction in which the mother organism produces offspring without the need for fertilization by a male. Spontaneous generation is a historical concept referring to the supposed formation of living things from non-living matter, while alternation of generations refers to the alternating production of sexually and asexually produced offspring in certain organisms.