As used in this chapter: “Commission” means a planning commission; “municipality” includes a city, town or borough or a district establishing a planning commission under § 7-326; “subdivision” means the division of a tract or parcel of land into three or more parts or lots made subsequent to the adoption of subdivision regulations by the commission, for the purpose, whether immediate or future, of sale or building development expressly excluding development for municipal, conservation or agricultural purposes, and includes resubdivision; “resubdivision” means a change in a map of an approved or recorded subdivision or resubdivision if such change (a) affects any street layout shown on such map, (b) affects any area reserved thereon for public use or (c) diminishes the size of any lot shown thereon and creates an additional building lot, if any of the lots shown thereon have been conveyed after the approval or recording of such map; “cluster development” means a building pattern concentrating units on a particular portion of a parcel so that at least one-third of the parcel remains as open space to be used exclusively for recreational, conservation and agricultural purposes except that nothing herein shall prevent any municipality from requiring more than one-third open space in any particular cluster development; “town” and “selectmen” include district and officers of such district, respectively.