Complex sentence: A chimera or chimaera is an animal that has two or more different populations of genetically distinct cells that originated in different zygotes involved with sexual reproduction; if the different cells emerged from the same zygote, it is called a mosaic.
Split into simpler sentences: A chimera or chimaera (British spelling) is a single organism (usually an animal) that is composed of two or more different populations of genetically distinct cells that originated from different zygotes involved in sexual reproduction. If the different cells have emerged from the same zygote, the organism is called a mosaic.