adj. Describes an animal or plant that has been genetically altered.
The transplantation of foreign genes into plants has provoked widespread fear around the world that mankind would poison the environment. But to date there has been less concern about transgenic livestock, in part because the reproduction of animals, unlike that of field crops, can be tightly controlled.
—Justin Gillis, “Cows and Clones on a Va. Pharm,” The Washington Post, February 28, 1999
The difference is that in natural selection the fittest automatically survive. In artificial selection we choose the survivors, and we may also arrange cunning hybridization regimes.

In genetic engineering we additionally exercise control over the mutations themselves. We do this either by directly doctoring the genes, or by importing them from another species, sometimes a very distant species. This is what "transgenic" means.
—Richard Dawkins, “Who's afraid of the Frankenstein wolf ?,” The Evening Standard (London), August 19, 1998
1981 (earliest)
The feasibility of producing such genetically transformed mice, which we call "transgenic" mice, depends upon several factors.
—Jon W. Gordon & Frank H. Ruddle, “Integration and Stable Germ Line Transmission of Genes Injected into Mouse Pronuclei,” Science, December 11, 1981