1. In bone marrow, a transplantation in which bone marrow and host cells exist compatibly without any signs of graft-versus-host rejection disease.
2. A condition or state of containing two or more genetically distinct cell types.

In humans, chimerism results from any organ transplant other than that between identical twins or from the mixture of blood cells between dizygous twins (either of two twins who developed from two separate fertilized eggs) in the uterus before birth.

A dizygotic twin refers to either of the "fraternal" twins who has shared a common uterine environment with its twin which is a result of a different fertilized ovum.

Having two stable, genetically distinct lines of blood cells, which can often be distinguished based on differences in blood group antigens.

In humans, this usually rises when blood cells from one dizygous twin mixes with the co-twin in utero; the organ in a woman’s body where babies develop before birth.