Determine risks of a new technology and then decide whether these risks are acceptable. Risk is mathematically defined as the product of likelihood of a certain outcome and severity of consequences from that outcome.
However, the problem is that we often cannot easily calculate the probability of certain outcomes occuring, resulting in uncertainty. Furthermore, we may not even know all possible consequences. This is called ignorance.
When an activity raises threats to the environment or human health, precautionary measures should be taken, even if some cause-and-effect relationships are not fully established scientifically.