Synthetic Biology is the engineering of biological components and systems that do not exist in nature and the re-engineering of existing biological elements; it is determined on the intentional design of artificial biological systems, rather than on the understanding of natural biology.
Definition from: Synbiology (2005) SYNBIOLOGY, An analysis of Synthetic biology research in Europe and North America European Comission Framework Programme 6 reference contract 15357 (NEST), October, 2005.