The following words and phrases shall, for the purpose of this act, have the meanings respectively ascribed to them in this section, except where the context clearly indicates a different meaning:
"Assessors" and "elected assessors" shall mean the assessors for county tax purposes elected in wards, boroughs, towns and townships in counties of the fourth, fifth, sixth, seventh and eighth classes.
"Appointed assessors" shall mean the assessors appointed by the board of revision of taxes in counties of the first class, and the subordinate assessors appointed by the board for the assessment and revision of taxes in counties of the second, second A and third classes.
"Base year" shall mean the year upon which real property market values are based for the most recent county-wide revision of assessment of real property, or other prior year upon which the market value of all real property of the county is based. Real property market values shall be equalized within the county and any changes by the board of revision of taxes or board for the assessment and revision of taxes shall be expressed in terms of such base year values.
"Board of revision of taxes" shall mean the board of revision of taxes in counties of the first class.
"Board for the assessment and revision of taxes" shall mean the board for the assessment and revision of taxes in counties of the second, second A and third classes.
"Common level ratio" shall mean the ratio of assessed value to current market value used generally in the county as last determined by the State Tax Equalization Board pursuant to the act of June 27, 1947 (P.L. 1046, No. 447), referred to as the State Tax Equalization Board Law.
"County commissioners" means the board of county commissioners or other similar body in home rule charter counties.
"Established predetermined ratio" shall mean the ratio of assessed value to market value established by the board of county commissioners and uniformly applied in determining assessed value in any year.
"High tunnel" shall mean a structure which meets the following:
72 P.S. § 5020-102