Mitigation banking is defined in the Federal Guidance for the Establishment, Use and Operation of Mitigation Banks Federal Register: November 28, 1995 (Volume 60, Number 228) Pages 58605-58614 as:

• Wetland restoration, creation, enhancement, and in exceptional circumstances, preservation undertaken expressly for the purpose of compensating for unavoidable wetland losses in advance of development actions, when such compensation cannot be achieved at the development site or would not be as environmentally beneficial. It typically involves the consolidation of small, fragmented wetland mitigation projects into one large contiguous site. Units of restored, created, enhanced or preserved wetlands are expressed as "credits" which may subsequently be withdrawn to offset "debits" incurred at a project development site.

For more info about mitigation banking, see:
http://www.mitigationbanking.org/projects.htm
Also check out the Rancho Jamul bank:
http://www.mitigationbanking.org/PDFs/CA-wildlandJAMUL.pdf
This comes up over and over in the mitigation sections of the 401s.-Richard