From Ethical Politics
"response to uncertainty, in the face of risks to health or the environment. In general, it involves acting to avoid serious or irreversible potential harm, despite lack of scientific certainty as to the likelihood, magnitude, or causation of that harm."
The precautionary principle has been a false friend to environmentalists. Caution is a political virtue, without a doubt. Its opposite is foolhardiness, and no one would want to be accused of that. But elevating virtues into principles leads to trouble.
As an example, Bjorn Lomborg delights in using the language of caution against the climate change agenda. He argues that the cost of cutting warming by using renewable energy sources in the UK may result in far worse outcomes than a small rise in temperatures---for example, those resources could otherwise be used to alleviate poverty whose consequences are also possibly very grave. 
Turning the virtue into the principle forces one to think about morality only as consequentialists. There is no future in that.
- The Precautionary Principle Project - multi-year study of the application of the precautionary principle to biodiversity conservation and natural resource management
- "The Precautionary Principle: A Common Sense Way to Protect Public Health and the Environment", prepared by The Science and Environmental Health Network, January 2000.
Author: Tony Curzon Price