There are several definitions for "agricultural use" and "farmland". The Farmland Preservation (formerly PA 116) program office use's the following definitions for Agricultural Use and Farmland:

"Agricultural use" means the production of plants and animals useful to humans, including forages and sod crops; grains, feed crops, and field crops; dairy and dairy products; poultry and poultry products; livestock, including breeding and grazing of cattle, swine, captive cervidae, and similar animals; berries; herbs; flowers; seeds; grasses; nursery stock; fruits; vegetables; Christmas trees; and other similar uses and activities. Agricultural use includes use in a federal acreage set-aside program or a federal conservation reserve program. Agricultural use does not include the management and harvesting of a woodlot.

"Farmland" means 1 or more of the following:
(i) A farm of 40 or more acres in 1 ownership, with 51% or more of the land area devoted to an agricultural use.
(ii) A farm of 5 acres or more in 1 ownership, but less than 40 acres, with 51% or more of the land area devoted to an agricultural use, that has produced a gross annual income from agriculture of $200.00 per year or more per acre of cleared and tillable land. A farm described in this subparagraph enrolled in a federal acreage set aside program or a federal conservation reserve program is considered to have produced a gross annual income from agriculture of $200.00 per year or more per acre of cleared and tillable land.
(iii) A farm designated by the department of agriculture as a specialty farm in 1 ownership that has produced a gross annual income from an agricultural use of $2,000.00 or more. Specialty farms include, but are not limited to, greenhouses; equine breeding and grazing; the breeding and grazing of cervidae, pheasants, and other game animals; bees and bee products; mushrooms; aquaculture; and other similar uses and activities.
(iv) Parcels of land in 1 ownership that are not contiguous but which constitute an integral part of a farming operation being conducted on land otherwise qualifying as farmland may be included in an application under this part.

Michigan property tax law also has a definition of "qualified agricultural property" that is to be used by local assessors to determine if a parcel can be classified for assessing purposes as agricultural. Click here to view the web site that contains a form used by the Michigan Department of Treasury regarding qualified agricultural property. The definition of qualified agricultural property is found on the second page of the form.