For the purposes of this chapter: “Commission” means a planning district commission. Planning district commissions are composed of the duly appointed representatives of the localities or Indian tribes which are parties to the charter agreement. “Indian tribe” means an Indian tribe or band that is recognized by federal law. “Planning district” means a contiguous area within the boundaries established by the Department of Housing and Community Development. “Population,” unless a different census is clearly set forth, means the number of inhabitants according to the United States census latest preceding the time at which any provision dependent upon population is being applied, or the time as of which it is being construed, unless there is available an annual estimate of population prepared by the Weldon Cooper Center for Public Service of the University of Virginia, which has been filed with the Department of Housing and Community Development, in which event the estimate shall govern.