From Envirowiki

An externality is the name given to any effect of a commercial system that does not have a monetary value put on it. A negative externality is such an effect that has a net negative outcome. Pollution, social breakdown and health problems are all considered negative externalities in the current market system, and as such don't rate a mention in discussions of economic policy.

[edit] Reducing negative externalities

There are two major ways to reduce negative externalities. The first, and perhaps simplest, is to put a price on the externality by law, either via market mechanisms, or via a tax, such as a pigouvian tax, or other fixed price. This can have a positive benefit, but is usually inherently flawed, because there are parts of most systems which can't Be reduced to an economic figure (human health, ecosystem services, extinction, etc.). Tools such as cost-benefit analysis can help, but fundamentally they are just making a bad system less worse.

The other main way is to base the social system on a different set of values, one not based on reductionist economics. This is pretty hard when the people you're fighting against have a vested interest in maintaining the current capitalist system, and have the financial resources to back up their campaigns.