A More straight forward definition of the Precautionary Principle

This definition comes form the ABC science web site glossary :

http://www.abc.net.au/science/slab/consconf/glossary.htm

This is probably the neatest most concise description I have found.

Precautionary Principle - The key principle of ecologically sustainable development. It basically states that in situations where there is lack of scientific certainty and possibility of serious or irreversible damage to the environment (both of which apply to gene technology), we should recognise this uncertainty; assess the possible impacts and our options; and put in place now whatever measures we can to avoid possible damage. We should not wait for clear scientific proof of harm.