1. Something that exists only in the imagination or is a creation of the imagination; an impossible and foolish fancy; and so it is not possible in reality: Economic stability in our country has become a chimera.
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2. In biology, an organism; especially, a cultivated plant, containing tissues from at least two distinct genetic types, often because of grafting; consisting of two or more genetically distinct cell types, produced as a result of mutation, transplantation, or grafting; the fusing of different embryos, or other similar processes: The fraternal twins were not chimeras as their blood types were different from each other.
3. In experimental embryology, the individual produced by grafting an embryonic part of one animal onto the embryo of another animal, either of the same or of different species: There was excitement in the laboratory when a successful chimera from the experimental transplant was announced.
4. A person who, or an organism which, has received a transplant of genetically and immunologically different tissue; such as, bone marrow: The multi stage process resulted in the successful transplant of bone marrow; the chimera who received the transplant is beginning to recover.

Human chimeras were first discovered with the introduction of blood typing when it was found that some people had more than one blood type.

Most of them proved to be "blood chimeras" or non-identical twins who shared a blood supply in the uterus.