72 Pa. Stat. § 5452.1a
The following words and phrases when used in this act shall have, unless the context clearly indicates otherwise, the meanings given to them in this section:
"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 shall be expressed in terms of such base year values.
"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.
"Established predetermined ratio" shall mean the ratio of assessed value to market value established by the Board of Property Assessment, Appeals and Review and uniformly applied in determining assessed value in any year.
72 P.S. § 5452.1a