Natural capitalism is a set of trends and economic reforms to reward energy and material efficiency - and remove professional standards and accounting conventions that prevent such efficiencies. It emerged in the 1990s as a coherent theory of how to exploit market systems and mechanisms of neoclassical economics to save energy, discourage waste, mimic ecology (biomimicry), and in general to support the goals of environmentalism by reframing commodity and product relations as a strictly service economy - extending the services of natural capital.
When capitalized as Natural Capitalism, the term usually refers to the specific set of reforms described in 1999 by Paul Hawken, Amory Lovins, and Hunter Lovins in the book of the same name. There is a well-developed theory of natural capital that predates this work, and some generic use of the phrase to apply to environmental economics. The book and additional material are available online at the book's promotion site: http://www.natcap.org/
A related, sometimes overlapping movement believes that networks are inefficient ways to provide services, and advocates autonomous buildings.