How do you define nature?

In dealing with the built environment it is not uncommon for us to ask ourselves “what is nature?” Do you separate mankind from nature? Is nature something that hasn’t been touched by mankind? or is mankind a part of nature and is it then natural for mankind to be manipulating ‘nature?’ To what extent? Steel skyscrapers with low efficiency? or a regenerative building with low impact and high efficiency? “Why is ‘this’ nature or natural, and ‘that’ isnt?”