I am not a big fan of definitions and terminology in molecular biology, because biology is not physics, and things that we might describe and define change, as does our understanding of them. However, as this question has surfaced again after a number of years, it might be worthwhile giving a more recent example of synthetic biology, rather than just defining it. That way the reader can get an idea of what it is about.
I would say that:
Synthetic biology is a particular type of genetic engineering.
A reasonable definition of genetic engineering is given by Encyclopedia Britannica:
The artificial manipulation, modification, and recombination of DNA or
other nucleic acid molecules in order to modify an organism or
population of organisms.
So this could include making a single base change to inactivate a gene or introducing and expressing the gene for human insulin in, say, yeast.
The definitions of synthetic biology are more contorted, e.g. that from the Royal Society
Synthetic biology is an emerging area of research that can broadly be
described as the design and construction of novel artificial
biological pathways, organisms or devices, or the redesign of existing
natural biological systems.
So why is the expression of insulin in yeast not considered synthetic biology? Basically because it lacks the complexity of a completely novel system. Consider the following description of an arsenic biosensor designed to test for arsenic in drinking water:
“Chris French at Edinburgh University led a team that turned the E.
coli bacterium into an arsenic sensor by rewiring two genes. One gene
senses arsenic and activates genes to pump it out of the cell; the
other allows the bacteria to digest the sugar lactose, producing
lactic acid. The rewiring involves putting the gene for digesting
lactose under the control of the arsenic sensor. When arsenic is
detected, the lactose-digesting gene switches on. The lactic acid it
produces makes the water more acidic, which can be detected using a
cheap pH indicator: if the reading is blue, the water is safe; yellow
means it is dangerous.”
So they have engineered a completely new system with two different genes, which have been modified and their regulation co-ordinated (“rewired” in the article) in an original manner. And there is a tendency to embody some of the complexity into modules that can be reused by others allowing even more complex and sophisticated systems to be constructed.
You could say that the difference from insulin expression in yeast is only a matter of degree, but here the devil is really in the detail.