Critical Infrastructure is an international term, hence this page should be internationalised to remove the US centricity. The term has been in common usage before Sept 11 and DHS. It is often synonymous with 'lifelines'. I'll try and find some more definitions and update the page in the future.
Aspect of Critical Infrastructure: Dependency Infrastructure
I do not have a dictionary definition for this term as it was encountered in a work of fiction but a good working definition might include the concepts: that infrastructure which upon completion and utilization, increases the population density which may be sustained in its field of operation, i.e. increases the safe population limit. Once growth occurs, this particular piece of infrastructure is necessary to the survival of the citizens depending on it.
Some of these items would most likely include such things as waste water treatment facilities and high-rise apartment buildings which are included in "Critical Infrastructure", but unlike some of the others mentioned, loss of these would inevitably lead to loss of life if not restored or the population density reduced (evacuation).
Unfortunately, much of engineering is foreign to me so I do not know if this is covered under another topic. But if its somehow possible, I would like to see a more scientific assessment of these items and perhaps some idea of at which population or population density, dependency infrastructure begins in earnest, at which point it accelerates significantly (dominates construction projects perhaps?) and to what degree these items add to the limit of population for an area.